C.Linton's right, but that's only a fraction of the story.
The split coil pickup has its two halves made and wired in an RWRP fashion - reverse wind (or reverse wiring) and reverse (magnetic) polarity. So one pickup half acts like it was wound clockwise, the other as if it was wound counter-clockwise. This has the effect of collecting the same amount of signal and noise in each coil, but since they're reverse to each other, the noise gets cancelled. However, since the magnets in each half are also reverse of each other (all north in one half, all south in the other), the signal (the actual motion of the strings) gets boosted instead while the noise remains canceled. That's why it works like a humbucker.
The second reason is that the offset coils in the normal position (EA half closer to the neck) enhance the spectrum of the bass' tone, giving lower lows and higher highs. On the other hand, some manufacturers put them in the reverse position (DG half closer to the neck) giving it a more even tone across the strings with brighter low strings and warmer high strings.
The third reason was, at the time the "tetris block" P-bass pickup came out, the humbucking patent was held by Seth Lover who made humbucker pickups for Gibson. So in effect, this was a humbucker that worked that way, but didn't appear like a humbucker.
Squier VM Bass VI club member #19