Quasi-connectivity doesn't always work the same

Yes, a bug is not working correctly.

In the screenshot you can see 4 pistons, 1 of which is extended. It is being powered by the redstone. It is on a slab, not a full block, so it must be from the redstone 2 blocks above it. This means it must be quasi-connected by the redstone itself. Since quasi-connectivity works from two blocks up OR one block up and one block in either direction, I would assume the other pistons to extend as well.

When the redstone is replaced with solid blocks which are all powered(from redstone on top of it), all pistons will extend(after a block update).

What I expected to happen:

The pistons to all extend.

What happened:

Only one of the pistons extended.