I think what's confusing is credits vs votes. It's more expensive in credits for Alice to cast the first N votes than to cast the next N. However, for Bob those "next N" are his first N. So 2N votes costs fewer credits if Alice and Bob each cast N than if Alice casts 2N.

Also, Bob might end up with "leftover" credits he can't allocate to issues he cares about. So those credits might go unused in some sense.