It's probably just an oversight. There wasnt ex+ when we got the first 15/35 weapon shop and i would wager good money that the display code from that first shop has been recycled for every shop since. No sense in reinventing the wheel, especially for game that constantly adds and removes content because the more you fuck with any existing program, the more likely you end up with bugs. Gacha games aint got time for bugs when every minute of unplanned maintenance is a minute that dffoo cant make money.

In game, ex and ex+ are the same weapon, but on the backend it was either faster or better to have the game count ex and ex+ as two distinct weapons because the ex+ has different game mechanics, e.g. so the game knows to show you ingots when you go to LB an ex+ or orbs if just ex. Making them separate was also probably the easiest way to try and save players from themselves by disabling the sell function for ex+.

Granted, im sure they could've fixed the ex counter in the shop by just asking the game to display both weapons tiers together, but someone would have to notice this little glitch AND report it. Idk if anyone besides you noticed the discrepancy, but even if there were dozens of you, is anyone really going to take the time to report a minor glitch to SQEX?