Why were the German naming conventions for its OOB so weird and senseless? It seems that many divisions corps, army, brigades, etc... had random numbers attached to them instead of just having them in order, e.g. 1. Infanterie-Division, 2. Infanterie-Division...

Same with the Americans too tbh.

The first conventions which gave the long-lasting and still-somewhat-in effect numbering scheme of U.S. Army units came into being in the summer of 1917, guided by the National Defense Act of 1916. To allow for any future expansion, the War Department reserved blocks of numbers for certain types of battalions, regiments, and divisions.

Component	Numbering scheme	Note
Regular Army infantry divisions	1-25
Regular Army cavalry divisions	15 and up	Excepting the 1st Expeditionary Division which deployed overseas in June 1917, the War Department could organize thirteen more Regular Army infantry divisions with the regiments still stateside
National Guard divisions	26-75
Volunteer Army divisions	76 and up

To conform to changing War Department policy and tables of organization, many National Guard units were broken up and redistributed, and all lost their state designations. Before the new War Department numbering scheme came into effect, the first National Guard divisions were numbered 5 through 20; they were later consecutively re-numbered to 26 through 41. As part of an expansion in 1918, the 9th through 20th Divisions were authorized, to be made up of Regular Army and Volunteer Army troops, as well as the 96th through 102nd Divisions, of the Volunteer Army. None of these divisions went overseas, and most were not even fully formed by the Armistice

The postwar reorganization of the Army initiated by the 4 June 1920 amendments to the National Defense Act of 1916 authorized the units of the National Guard and the Organized Reserve (the combination of the Enlisted Reserve Corps and the Officers’ Reserve Corps created by the 1920 act) to keep their names bestowed upon them by the War Department during World War I, thus essentially continuing the numbering order already set down.

SEC. 3a. THE INITIAL ORGANIZATION OF THE NATIONAL GUARD AND THE ORGANIZED RESERVES.--In the reorganization of the National Guard and in the initial organization of the Organized Reserves, the names, numbers and other designations, flags, and records of the divisions and subordinate units thereof that served in the World War between April 6, 1917, and November 11, 1918, shall be preserved as such as far as practicable.

Until the above amendment was passed and War Department inspectors could get around to interpreting it and allowing units to be redesignated, National Guard units were reorganized temporarily under their old state designations. The 134th Infantry Regiment;

Arrived at the port of New York on 24 January 1919 on the U.S.S. General Goethals as the 134th Inf., an element of the 34th Div. Demobilized on 18 February 1919 at Camp Grant, IL. Reconstituted in the N.G. in 1921 and allotted to the state of Nebraska. Concurrently relieved from the 34th Div. and assigned to the 35th Div. Reorganized on 25 October 1921 by redesignation of the 1st Inf., Nebraska N.G. (organized 1920–21; HQ organized 7 July 1921 and federally recognized at Omaha, NE) as the 134th Inf.

The Army decided in peacetime to maintain nine Regular Army infantry divisions (the 1st through 9th), two Regular Army cavalry divisions (the 1st and 2nd; although the latter was constituted, the headquarters was only activated in 1940), three territorial infantry divisions (Panama Canal, Hawaiian, and Philippine; the numbering of the Regular Army units assigned to these divisions corresponded to those which would have made up the 10th, 11th, and 12th Divisions, respectively), eighteen National Guard infantry divisions (the 26th-45th, originally less the 31st and 42nd), and twenty-seven Organized Reserve infantry divisions (the 76th through 104th). The National Guard and Organized Reserve were also allotted cavalry divisions, but these do not have any bearing on the discussion here. A National Guard division's headquarters was not permitted to be organized and federally recognized until at least 75 percent of the division's subordinate units had been organized and federally recognized.

The allotment of National Guard divisions to states as it had been in World War I required some reconfiguring; the 43rd, 44th, and 45th Divisions were new units. The 42nd Division was not reformed, since it was a special case made up of National Guard units from 26 states and the District of Columbia. The numerical designations of three divisions, the 30th, 31st, and 39th, were offered to the planners in the Fourth Corps Area (Alabama, Florida, Georgia, Louisiana, Mississippi, North Carolina, South Carolina, and Tennessee) as these divisions had been organized in that area for World War I, and the 30th and 39th were chosen, with the 31st Division being the second division deleted from the mobilization program.

The 39th Division initially was allotted to the states of Alabama, Arkansas, Florida, Louisiana, and Mississippi, but before the headquarters was organized and federally recognized, the adjutants general of the states assigned to the division petitioned the War Department to change the designation of the division to the 31st Division (in World War I, the 31st Division was made up of troops from Georgia, Alabama, and Florida, while the 39th Division was made up of troops from Louisiana, Mississippi, and Arkansas). The 39th Division, less the Arkansas elements which became non-divisional troops, was redesignated the 31st Division on 1 July 1923.