The wiki makes more sense.
BMC is a weird term and it doesn’t fit in with how it works AFAIK
Basically, you have 100 “points” (default) and it costs 100 “points” to move on normal land.
If you have 99 speed, you will actually cut into the first turn but then have enough speed to move normally for another 99 turns.
If you had, say, 150 speed, and a weapon with a 50 cost, you could theoretically move and attack in the same turn (with no penalty). The reason 150 speed is faster per se is because you can move once, and then every two turns you can move twice. It’s a win-win situation.
Meaning, logically, even if your speed is one above an enemy’s speed, eventually you will catch up, and vice versa.
Now, the bushes, fences etc. have movement costs higher than 100. So, if you have 100 points and are moving through a clean window (400 cost) you will have four turns of inactivity as you move onto it. You will not get a penalty solely for being on it, however.
The reason one-space windows work so well is because for the first 4 (or more, it depends) turns, the zed is still moving onto the tile, and not attacking you. Theoretically, though, after that grace period, it would attack normally; it depends on how much it costs for a zombie to attack (I would guess 100) and its speed (according to the wiki it’s 70) means that it can attack on most turns, but not all. This is the “slipping by” effect we see sometimes.
However, it also means that it’s almost completely random when you get attacked by a zombie, or how fast something moves. Because you can’t be in-between tiles, when something is innately faster than you it actually just “randomly” moves two or three tiles instead of its normal. If you were a zombie viewing the player character, the same effect would happen; sometimes you could attack, but other times the human would completely outpace you.
Speaking of which, a zombie mode could be interesting…