Found the problem.

"this is how I operator from April through August and you get better work from me because of it"

BFX wants to go climbing every Friday from April to August. He tells his employer "I'm out every Friday from April to August but I'm gonna work extra-special hard Mondays and Thursdays to cover it" and his employer, thinking BFX is an awesome get, goes along with it.

BFX's group needs to get on one page so they schedule a weekly standing meeting. It can be any day of the week, except April to August, so now it's going to be Thursdays. Except we discover that BFX is busy clearing his schedule half the year so he can go climbing so it should probably be Wednesdays.

BFX works with a nice lady, CGY. CGY has PT twice a month from a nasty fall she had last winter and she'd like to do it afternoon Fridays because her PT's office is the other direction from home but she can meet her girlfriend at this nice dim sum place after. Except Fridays are already skunked 'cuz BFX might be off climbing a mountain so now she does it Thursdays.

They've got a coworker DHZ who's keeping it together through AA meetings. He'd love to grab them during lunch but Thursdays and Fridays are now out which gives him Monday through Wednesday.

And it all works and everyone's schedule is all fine until there's wind in the pass and BFX comes in on Friday anyway and he's super-productive because he went climbing last week and it was dope. CGY is a little put off because she totally could have done her PT today except she did it yesterday. Maybe she could do it next Friday instead of next Thursday? Except nope, because we don't know what the weather will look like. But since we're all here let's get our Wednesday meeting out of the way and boy howdy it works better for everyone! Except we can't rely on this because BFX is probably climbing a mountain next week unless he's not.

To you? It's a day that you can take off, or not take off, and it's no skin off anyone else's nose. To the rest of the office it's a day whose composition will change on your whim. You'll notice I didn't even get your boss involved, who you think is the only other person in this equation but lemme tell ya - CGY and DHZ aren't gonna bitch to you. They aren't going to tell you about their PT appointments or their AA meetings. They're going to tell the person who needs to make sure the machine keeps running no matter what parts decide to go alpining.

What would somebody do with their PTO if they had plans and had to burn it because of bad weather, conditions, etc.?

SOMETHING ELSE.

It's easier to tell CGY "BFX is going rock-climbing on Friday is there some other day you can do your PT?" than it is to tell CGY "BFX reserves the right to maybe go rock-climbing on Friday so you don't get to do your PT that day." "one person isn't coming in" is hella easier to deal with than "one person might not come in" because nobody wants to be told "I MIGHT need you to change your plans, depends on what the weather is for someone else's time off."

If you've got time off scheduled, and the mountain isn't cooperating, go watch a movie. Go on a road trip. Visit a museum. Don't keep your coworkers guessing.