All divisions are not created equal. in 1917, most British Empire/Commonwealth forces underwent a restructuring that made them smaller.

This was proposed for the Canadian Corps as well. The Corps would have gained a 5th division from Canada (under Garnet Hughes, the son of notorious Minister of Militia and Defence, Sam Hughes,) as well as 6th, to be made from reducing the standard Canadian division from 12 battalions to 9. This would have made a Canadian Army, composing two corps of six divisions apiece. Each division would have had roughly 3 brigades of 3 battalions instead of 3 brigades of 4 battalions. To put it more graphically:

CANADIAN CORPS (ORIGINAL) Corps Troops

4x Division Divisional Troops

3x Brigade Brigade Troops

4x Battalions

PROPOSED ARMY STRUCTURE

Army Army Troops

2x CORPS Corps Troops

3x BRIGADES Bde troops

3x BATTALIONS

Now why is this important? Two reasons:

1 - Shock, Casualties and Depth

By late 1917, it was abundantly clear that the war was causing a massive number of casualties - hence the press for the reorganisation in the first place. Importantly, this was happening on both sides.

The Germans responded by concentrating their most battle-worthy troops into assault divisions and their second-rate troops into trench divisions. This concentration of potent forces, with depth and reserves (in plain English, troops that are kept a bit further back to be called up later,) allowed concentrated forces to break through on a narrow front and exploit successes. Well-trained, motivated troops finding weak points and breaking into them with the co-ordinated use of new weapons and artillery, then push through those points helped by with readily-available, and most importantly local, fresh, follow-up troops almost immediately.

Although the German offensives of March 1918 petered out for a number of reasons (let's save that for later,) they demonstrated the importance of keeping units intact so that reserves were from your own battalion/brigade/division. This examples validated Currie's determination to keep the Canadian divisions strong, despite the fact that he would have gotten a promotion to a full army commander had he reoganised them.

The German offensives had also been hard on a lot of the British formations to the north. One group they had not been hard on was the Canadian Corps, which had largely been spared from the fighting. These weren't any four divisions. These were four full-strength divisions who had not been bled dry during early 1918.

2 - Morale, Cohesion and Effectiveness

The constant reogrnaisation of a lot of the British formations broke up long-service units and the constant shifting meant that units were constantly getting used to new staffs. The Canadian Corps, however, was made up of units that had largely stayed together since their formations and were used to each other. This makes a huge difference when you need to quickly organise and plan attacks in order to maintain momentum - the kind of fighting that went on during Hundred Days. Smooth, seamless integration of artillery, attacking troops, reinforcements (and to a limited degree, tanks.)

In this case, they weren't any four divisions - they were four strong, experienced, cohesive, well-manned ones. The troops knew their leaders, and their leaders knew their staffs, and their staffs knew their jobs. So did the soliders. They knew and understood new tactics, meaning that they could maintain their momentum. This kind of knowledge and responsiveness allows formations to keep moving forward without the need to stop and undertake an extensive planning process, or call for reinforcements from the outside.

The divisions were the kind of striking force needed to crack the tough nut of the Germans holding on to the ground of their choosing.

Sources:

Tim Cook, Shock Troops: Canadians Fighting the Great War 1917-1918 Tim Cook, The Madman and the Butcher: The Sensational Wars of Sam Hughes and General Arthur Currie

For a good understanding of the German Spring Offensives and their tactical development, See: Bruce I. Gundmonsson, On Infantry