"Storage card" is your internal storage, or more precisely it's the part of the internal storage that's available to the user for apps, data and media. This is historical in origin, but the user-accessible part of the storage can be viewed as if it were a storage card (a decade ago you'd find references to an "internal SD card", which was really a partition in the internal storage and not an SD card at all).
So what it tells me is the size of your actual system partition, and that that isn't growing. This "user volume" of the internal storage is 10934 MiB in size, where "MiB" is used to denote the binary definition of "MegaByte". This is equal to 10.6 GiB, while your phone has a capacity of 16 GB (decimal) = 14.9 GiB. So that tells us that your system partitions total 4.3 GiB in size (or 4.6 GB in decimal units), which is pretty consistent with your U11.
Unfortunately that doesn't solve the problem, it just tells us that the actual system partition is not growing in size. The problem is that 6215 MiB (= 6.07 GiB or 6.52 GB) of "system data" you can see in the second shot. My guess is that that is being added into the "system" usage in your storage menu. The trouble is that without root you can't see what is actually in there, so that doesn't help us understand the cause or fix it.
Did you get a lot of app updates after doing the factory reset? My guess would be that any system app that was also in the Play Store would probably be updated, since the ROM is bound to contain old versions (as it's old itself), and that would contribute to some of this. But it's hard to believe that adds up to 6 GB, and that wouldn't keep growing with use. You've ascertained that caches don't count for a lot of it, so that only leaves system app data (which you probably don't want to erase). But it does still seem like a lot.
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