It seems like you should be able to do 39 hours 1 week and 41 the next, as long as it is in the same pay period, but no that is not the case.
You need to not necessarily DO the 40 hours or whatever you are scheduled for, but you have to account for them, to my knowledge, either with PTO or OOW accounting for a shortfall. This is why when I have OOW I do the protocol so in case I am short hours I don't have to use PTO unless I want to.
Not sure why this is really, but I recall a big push on this a few years ago or something, so I always make sure I square up the hours every week.