Yup, those corners are to fit small islands to the most sensible zones according to nearby continents.

There's more though. I think there are a few zones within other zones. There are a few half offsets. And more like that.

I think one island (or something) moved from the one half (+x) into the other half (-y) or vice versa. It's quite a bit of fun if you only need to enjoy reading an explanation and not have to work with these anomalies.