The height from floor to top of keys appears to be standard but appears to vary from floor to underside of keyboard. On my acoustic piano it is 24in. but on the digital 25in. I have tried highering the acoustic piano with wooden blocks by 1.1/4in but then the pedals are too high so I have compromised by having the blocks 1/2in. My inside leg is 28in. from floor to knee and I find to have the knees underneath the keyboard cramps the leg movement and restricts the pedal swivel on the heel. I think I will have to make-do!
Can't think of anything except you're sitting too close to the keyboard. When one sits at the piano, one's hands and thus fingers need to be comfortably in the proper place to play. This puts the elbows vertically in front of the hips, and moves the body away, so the knees don't have to be under the piano.
Just had a lesson with a 6'+ student - still growing! Floor to knee top 25", won't quite fit under the piano, but doesn't need to, as long arms go with long legs, and the knees are always outside the keyboard. Longish feet mean he can reach the pedals sitting like this. Where's the problem?! Sit further away, with straighter, but relaxed arms.