"Except" is biconditional, but why not just Group 3?

SurfandSailandFly

I do not understand why 7Sage teaches "except" as a biconditional. It makes more sense to me to see it as a simple Group 3, where you just negate the sufficient. I've seen other programs teach it this way, too. I'm not trying to get into a comparison argument. I just want to know which is going to be useful to me on the LSAT. Here's an example: "I go to the store, except on Monday." It seems that the aforementioned sentence is the same as "I go to the store, unless it's Monday."