Do make allowances for artistic license, composition etc. Anyway, the Roman writings on the phalanx make a note on the staggered spear-points as well, and a fair bit can be deduced from considerably later Europan and Japanese pike formations - there being only so many ways to use the damn flagpoles right.

Plus, shorter pikes for the front-rankers would be not only logistically annoying and generally pointless, but downright tactically stupid - when the whole point of the weapon and formation is to out-reach the enemy with serried ranks of long pointy things, so that he is held at bay and hopefully dispatched without ever having the opportunity to land a blow himself, shorter shafts would only be counterproductive.