Well, looking at a base stat total is one thing, but then you should compare it to moves and abilities, and you might find it cancels off. These Gen 5 starters do have great STAB moves, as well as abilities. For example, if Serperior used Leaf Storm and had the ability Contrary, the increase in SpAtk can definitely make up for the stats, in which you may find is a little lower than usual.
But, to answer your question, it's ultimately Game Freak's decision to make it like that. If you look at other generation starters, you might notice that grass Pokemon have a lower base stat than the other two typings, but that's just a random finding. Also, if you look at Gen 4, Torterra has a base stat total of 525, but Torterra is overall a powerful Pokemon. Game Freak doesn't want all the starters to have the same base stat total, so you will assume some Pokemon are weaker bases off of stats - but you should make a decision based off of the whole picture.
Hope I helped! :)