Why does overtime always start after your shift? And not before it?

My husband is currently working an 11am to 8pm shift. (more or less). His travel time is about 1.5 hours. Because he is the low man on the totem pole, he is subject to mandatory overtime.

Great! More money for us.

Not so great, his overtime shift ends at 11 or midnight. That’s not great for us. It’s bad enough he gets home after 9pm. It doesn’t leave any room for family time, and our daughter suffers for that.

So it’s a general question? Why is scheduled overtime always at the end of one’s shift?