The 1x1 patches inside your resistance areas is actually a bug I'm trying to work out, I may have it fixed by InDev 30d, but it could be pushed off to InDev 31.
Check under the data view, there's a "corruption resistant" data view. It'll show you how much and how strong your resistance is in certain areas.
The game currently does a poor job of explaining what "power" the corruption currently is (in part, this is because some missing mechanics coming into InDev 31, so I didn't make a GUI for it). But basically every game day the amount of "permanent" corruption expands that you can't do anything about, but the permanent corruption tiles can be less than the total that exist, if you trap the corruption and it has nowhere to go. Once that happens for every 1,000 tiles the permanent corruption tile count is higher than the actual amount of tiles on the map, the corruption power increases by 1. As soon as it increases by 1, every tile with a corruption resistance value of 1 becomes useless in pushing back the corruption, but values of 2 or higher still do. Eventually another 1,000 can tick up, pushing it to 2, then 3, and so on. So the longer the corruption is trapped, and the smaller the area it's trapped in the more powerful it gets, and the weaker your resistance is.
Corruption is corruption, it's all the same. You just can't go lower than the current permanent corruption amount if it has already spread. For example, if the permanent amount is 2,000, and the corruption covers 3,000 tiles on the map, you can push back 1,000 tiles of it (any 1,000), but once the count drops below 2,000 your efforts will be in vein. This plays into what I said up above, because say you push it back 1,000 tiles and trap it, and now only 2,000 tiles exist and it has no where to go that's when the corruption power starts slowly increasing day by day. If the permanent value reaches 3,000, even though only 2,000 tiles exist, it'll start busting through resistance areas that are only level 1.
Finally, corruption resistance does stack, it's designed that way on purpose so you can make highly resistance barriers with enough time.
It's very complicated on the code side, I plan on making a GUI in InDev 31 that explains all this in easy to view numbers/values. For now, none of what I explained is visible anywhere in game.