If I had to guess, its due to my implementation of it being weird. Currently, when you do your first jump it activates a jump cooldown for ~0.2 seconds. After that cooldown, you can double jump. When you double jump, it takes your velocity.y -= DOUBLE_JUMP_FORCE. Because of that, the double jump ends up noticeably more powerful when done early in the jump vs later when gravity starts taking effect.
Once the judging period is over I may test resetting the velocity.y when double jump is activated, then do the calculations. Should make it more predictable.
Thanks for checking it out!