youre subtracting out the change in shares outstanding, not neccessarily subtracting out share repurchases as your title suggests. ( - Change in shares outstanding)
If the shares outstanding increased, that would mean the company issued new shares (a positive increase in change of shares outstanding) and thus you would be subtracting from expected returns as people paid into the stock and thus brought down returns.
However, if shares outstanding DECREASES, meaning the company repurchased shares like S2000 said (a negative decrease in change in shares outstanding), that means that you will be subtracting a negative change in shares outstanding, and of course two negatives makes a positive….thus increasing expected return as shareholders received cash from the companies.
Does this make sense??