Z moves faster than x and y

hi everyone, I 've got a weird case i can't get it solved, would like to have some inputs.
While building up a delta-style printer with ramps 1.4, and marlin, Z axis always moves faster than the other two.
It seems moves more than double the distance than x and y, while x and y moves the same. I've switched them, and it doesn't help, whichever connected to Z port moves same amount faster.
Any one would suggest what's likely the issue?