This would only be the case if policy were linear, which it isn't. ACA is a good example of this. I forget what the exact count when it passed in the House in

This would only be the case if policy were linear, which it isn't. ACA is a good example of this. I forget what the exact count when it passed in the House in March 2010 was, but Pelosi had several "backup" votes to spare. Kucinich, for example, voted against the bill in protest, but if they had known that the bill had less support, she could have counted on his vote towards passage. Whipping votes takes resources - you don't want to use up any more favors than you have to, because that means one less favor you can call on for the next bill. It's better to call in exactly as many favors as you need to guarantee passage, and then let the rest slide so you can maintain leverage against them next time.