Internal storage drives me nuts.
My 8GB Moto E 2 was around £80 as well, so this gives us a reasonable actual real low-ish end phone to get numbers from. The phone reports that the OS (Marshmallow, patched to December 2016) occupies 3GB. I've no idea whether that is true or not, but that's the space allocated to it. That leaves 5GB. Currently it is reporting 4.15GB in use, or 83% full. I've also got a 16GB SD card in the non-portable mode, whatever it is called.
Looking at the internal storage, it reports 1.95GB being used by apps and 454MB for cached data. There are no other categories. Where is the other 1.75GB? No idea, but something seems to be using it because I do get the "not enough space to update an app" message occasionally. Part of it will be down to app updates for the provided google/motorola apps that can't be removed - seems like a huge amount to be missing though. Of the 1.95GB that is being used by apps, 1.54GB is related to apps that I'm cannot uninstall or move to external storage (admittedly including lots of things I want like "chrome", "phone" alongside the stuff I don't want like play news... )
The external storage has 630MB of apps plus images, video, audio, other and 130MB cached data. The majority of the apps can't go on the external storage, I've already done as much as I can.
If I installed all of the 38 apps (at an average of 6MB each - that would be 228MB...) that I've installed in the internal storage, I'd have used 4.15+0.63+0.13 = 4.91GB and have 90MB spare.
38 may be nearly twice the 20 app bar you set, but the ones that put me into the geek category like google authenticator, andftp, juicessh and remotemouse are all tiny. The apps that the less geeky people are installing are likely to be facebook, instagram, whatsapp and they take up loads of space, so I don't think I'm that far off from a "normal" user in that regard.
So I put it to the jury that Mr. Bacon is full of bovine do-do.
By the by, mean average isn't a very useful measure when there are lots of outliers. I'm sure the size distribution of apps will be very skewed towards small apps.
All the best,