Exactly. A single bump on each leg might convey a slight advantage, and then the creature that gets two pairs of bumps that mesh together would be even better off. From there it's just a matter of time before creatures with multiple bumps emerge.

Being able to jump quicker than your less-able siblings could convey a huge evolutionary advantage. This quickly weeds out the less-able and strongly encourages this kind of structure to develop once the process has started for whatever reason.