make key velocity response same from bottom of key to top of key on keyboard

katinka, I saw this post lingering, but you pose a legitimate question.
I'm not a piano player, but I have had some minimal experience with pianos. I am guessing by "the top edge", you mean closer to the pivot point? If so, then yes, it takes considerably more force to get the same swing from the keys. This, of course, has a dramatic effect on how velocity might be interpreted.
The A-500 has a synth action keyboard, as opposed to weighted keys. While the synth action is suitable for triggering synths, many piano/keyboard players seem put off by it, because it does not feel like a piano.
Understandably so.
For my purposes, I use the controller to trigger mostly non-piano instruments. To this end, it is very well  suited, and I find weighted keys somewhat distracting when playing non-piano instruments. However, I can fully understand how you would want it to feel like a piano.
From where I sit, you have two choices.
Adapt to the synth action keys, or consider a different controller.
I fully understand the importance of feel in an instrument, which your controller is.
I hope that is helpful in some way.