Actually most straight infantry corps don't gain any strength when they form up. German, British, and US 6-4 corps are formed from three 2-4 divisions each. British 4-4 corps are two 2-4 divisions each. The three 1-2-4 German divisions that make up a 4-6-4 corps gain one attack factor when they form, but mostly because there is no simple way to divide a 4-6-4 corps in thirds.
Remember that the game is a corps-level game, not a division-level game. The strengths we work with most often are corps strengths. Armies are multiples of those (with some modifications) and division are fractions of them. Since those fractions don't always come out even, we tend to round down on the division strength, because we don't want a corps to gain strength when it breaks down.
FYI, here is Frank's reply:
Actually most straight infantry corps don't gain any strength when they form up. German, British, and US 6-4 corps are formed from three 2-4 divisions each. British 4-4 corps are two 2-4 divisions each. The three 1-2-4 German divisions that make up a 4-6-4 corps gain one attack factor when they form, but mostly because there is no simple way to divide a 4-6-4 corps in thirds.
Remember that the game is a corps-level game, not a division-level game. The strengths we work with most often are corps strengths. Armies are multiples of those (with some modifications) and division are fractions of them. Since those fractions don't always come out even, we tend to round down on the division strength, because we don't want a corps to gain strength when it breaks down.