>>53778271Or far more likely (ignoring obvious dex hyperbole when it comes to outlandish feats like supersonic speed) Speed is a measure of initiative, responsiveness and acceleration rather than top speeds.
If Garchomp acceleration is like a jet taking off, this makes more sense.
Onix is on the flipside a snake in the 'ready to strike' position by default, which is a world of difference and easily arguable in the "110-130 speed" range.