Here's another question I get pretty regularly when answering our bug reports: what's the deal with those "vs LH/RH Starter" platoon splits? To wit:
"How can Reed Johnson hit 32 HR's off of LH Starters when he hit 27 vs LHP?"
At a glance, it seems like we're saying "this is the player's stat line just against the opposing starter, broken down by that starter's handedness." In which case it would seem to be a bug when a guy has bigger totals against, say, lefty starters than against lefties in total.
Except that the definition I just wrote is not at all what that split is measuring. Instead, the "vs LH/RH Starter" split adds up all stats accumulated in games where the opposing starter was of a certain handedness, INCLUDING STATS ACCUMULATED LATER IN THE GAME WHEN THE STARTER IS PULLED, REGARDLESS OF THE RELIEF PITCHER(S)' HANDEDNESS.
Despite bolding, italicizing, and going all-caps, I still don't think I emphasized that enough. I realize the description of the split seems like it's talking only about stats accumulated against the starters, but it's really just counting up all stats in games where the opposing starter threw a certain way -- a BIG difference. If you want to know about performance against just starters of a given handedness... well, that's a double split, so we can't answer that right now. But we do hope to add the capacity for double splits in the future.
Anyway, now you might be saying, OK, well what's the point of that split, then? It seems pretty useless. And in some ways it is, for recent seasons at least -- but it's the only way we can approximate platoon splits for pre-PBP era players (1940s and before). Even going back to 1916, we at least know who started each game and their handedness, which usually is good enough to get platoon data on the majority of PAs (especially before managers started aggressively using relievers).
So that's why that split is there, even if it's not especially useful for 2013 players. And now you know what that split means, even if it seems like it should mean that other thing (which it DOESN'T).