I've noticed the same principle in my pushup exercises.

If I say that I want to do 30 pushups then when I'm at the 30 mark I'm so tired that I can't even do another one.
But if I am to push my limits and expect more than what I regularly do, or go with an attitude like "I'm going to do as many as I can", I will always do a little more of them if I don't limit myself to a certain number.

Although different examples, I think the same principle applies.

Great post @KaylinArt! Thank you for sharing :)