I have gotten a world to 1% corrupt and have been hunting down small bits of corruption for a while now, and I'm wondering if demon alters or demonite count towards the dryads calculations and if anything else does that I might be missing.
Tiles that count towards the total corrupt count include: Ebonsand, Ebonstone, Corruption Thorny Bushes, Purple Ice Blocks, and Corrupt Grass. Look for these; I also suggest drilling holes every 40 or so tiles, going straight down to the Underworld. Then, use the Clentaminator in both directions when falling down the holes (go slowly!). If you are pre-1.2.3, I suggest removing any Red Stucco from the world. I'm not sure if this counted towards corruption counts, but it used to (pre-1.2.3) spread corruption.
As far as I can tell altars (both crimson and corruption) do not count as corrupt (or crimson). However, any time an altar is broken, there is a 66% chance that a single random tile in the rock layer will become ebonstone or crimstone (depending on the world) or pearlstone. These tiles can create an entire biome if left unchecked (which might be your problem).
Corrupt thorny bushes do spread corruption and must be destroyed as well (another possible problem). Other corrupt plants do not spread corruption as far as I can tell, but the block they rest on might.
To make clearing things out easier, try investing in the Clentaminator.