Why is TimeHome the only Hub Variable listed as in use? i.e. why doesn't the UI list the Apps using lastFridgeTemp and sonosHTTPAPI?
Okay, I see now that if I click on a variable name, the UI will tell me where it's used but other variables that are used by Apps aren't hyperlinked so they can't be clicked...
Looking at your variables it appears that lastFridgeTime, xxxxTimeHome, and season all have applications that have registered as using them. As @Inge_Jones said above, it is up to the application to register their intent to use the variable.
I didn't intentionally configure Apps to register their use of any variable.
I just create variables and use them in Apps when necessary. My expectation is that if a variable is used by an App(s), that there is a way to know which App(s) are using that variable.
This isn't your job; it's the app's job. Nothing you can do if the app doesn't register it. (IMHO, it's something an app should do, but apps do have to do some work to make this "registration" happen, as @thebearmay notes. Unlike with "in use by" devices, the platform can't automatically register variables as in use--due to differences in their implementation.)
Presumably, the built-in apps should do a good job at this, but if you notice something that isn't registering as you expect, consider posting about it here to see if someone can replicate the problem. Staff can probably address it if it needs fixing. If you're using a variable in a community app, consider letting the author know that you would find this "registration" useful (for example, you could post in the thread developers often create for discussion about their apps).
Along those lines, I'd guess that built-in (or community) apps that let you build your own strings like "The value is %myNumberVariable%" are probably not registering themselves (guessing it's mostly ones that present them in a drop-down), and I can understand the difficulty in doing so from the developer's perspective...but it would probably still be good if they could do so. [EDIT: Apparently, they are! So, if you notice something that isn't working, I'd again just note what app and how you've configured it so it can be addressed.]
That information is aimed at developers, not end users--so nothing you can really do if you're using a built-in app (or don't feel like modifying community or self-written code you might be using).