I assume you mean stuff like "first black senator since reconstruction" and the answer is that in the early days of reconstruction the federal government stepped in to ensure the physical safety and voting rights which allowed blacks to vote and thus elect not only republicans in the south put black republicans into office. Look at this CRS report on blacks in congress

http://www.senate.gov/CRSReports/crs-publish.cfm?pid='0E%2C*PLW%3C%20P%20%20%0A

page 2 graph: (40th congress =1867-69)

as it shows "No African Americans served in Congress from the 57th Congress (1901-1903) until the 71st Congress (1929-1931), when one Member was elected to the House. "

This is even more extreme when you consider southern black representatives where none served in congress from 1900 to 1970 (there is a fascinating history concerning how arguably "since reconstruction" is sometimes wrong since a literal handful of southern black representatives were able to get elected in the initial years after reconstruction ended but before the suffrage door was fully shut. we should ignore that though for the sake of the clear intent of the statements), and in the senate where after 2 early mississippi senators another wasn't elected until the 1960s anywhere in the country.

so "since reconstruction" is used because as reconstruction protected black suffrage a decent number of black representatives got elected to congress especially in the south but after the post reconstruction crackdowns on black suffrage there were 1. literally decades without any black legislators and 2. the early post reconstruction black congressmen really were rare, unique and deserving of honors that go with being "the first" as they were going against the prevailing winds.

to use a sports example: even if we found out that some early baseball players in say 1890 were black, Jackie Robinson would still in some sense be the "first" black ballplayer. Indeed I'm thinking of someone like [Moses "Fleetwood" Walker] (http://sabr.org/bioproj/person/9fc5f867) in the American Association (a short lived major league competitor to the national league).

as always with questions concerning Reconstruction. Foner's titular book remains an excellent source (i may come back and edit the book if i get home and get a hand on my version).