No particular reason.

Should it be 2 or 4? I'm guessing 2.

Hmm... If I did that, I would want the distance traveled due to the jump to change with each multiplier.

EDIT: Considering some adapted rules I made, the character now no longer doubles his speed base speed until level 13, he can merely go twice as far. In addition, the damage is divided by four. Under this, before level 13, he has a +4 bonus due to speed (+8 if we take Quick). At level 13, he gains a +20 instead (28 if we take Quick) This basically increases his damage from 1 or 2 to 5 or 7. If we consider items and stuff, none of these double because they are not "Base", so 4 due to ranks, 0-5 due to the d20, 2 due to feats, ~5 due to items. Does this sound reasonable? Alternatively, we could do /2 instead, since I misunderstood the charging rule. 8 due to ranks, 0-10 due to the d20, 10 due to items, 4 due to feats, so somewhere around 20 average damage.