Interesting, I never noticed it, but it seems you're right. I looked at the local storage for the itch.io address (https://v6p9d9t4.ssl.hwcdn.net/), and there seem to be entries for a lot of different games (including three of my own). This might explain some storage-related bugs that people are reporting. I notice one key (not from my own game) is just "gameJSON" - that might easily cause some clashes!
I guess the conclusion for now for developers is to use better keys, like gamename.valuename instead of just valuename, and to keep the size of the data in mind.
By the way if you want to test: I have three WebGL games that use localStorage: Ostark & Didi, Robo Repair and Trading with Werewolves - just click on my profile to find them. (They only add very little data to the local storage, so don't worry about the 5MB limit.)
EDIT: this does open up some fascinating possibilities for cross-game progress: for instance, if you unlock the FIREBALL in Vampire Survivors, you get it in my game too... :-D
EDIT2: ...or if you're up for using some very questionable marketing strategies: "play my game - as a bonus you unlock all characters in Vampire Survivors!" Hm...