“One banana is one banana though. So where is the sense?“
It’s still got one banana calories, but it’s smooshed into a smaller size, so less filling, you’re not chewing it, so less filling.
You might not add anything to it because bananas come in ones, but you may well go, hmm, I’m still hungry and have something else or you might eat lunch a bit earlier and then eat something else you wouldn’t have after or you might have a bigger lunch than normal (no clue why I’m assuming the mashed banana is in the morning btw).
The calorie density and how filling is part of how things are free, fruit only works with that if it’s being eaten in its natural state, otherwise it tips into being a risk to your calorie deficit.
If you’re 100%, you’re eating a third or more of the speed free stuff, you’re making no mistakes with your free stuff, you’re well within syns and your healthy extras are properly weighed or measured... a banana being mashed isn’t going to make much difference.
But, if you’ve got a few little other things going on that aren’t quite how it’s supposed to be - then it’ll add up and it might.