Well first of all the control panel applet has a tendency to lie and isn't very accurate at tracking used memory - it's entirely possible to run out of physical flash storage (so that it's impossible to install any further applications) and the control panel applet will blissfully inform you that there is still tons of memory available. Not gonna be fixed it would seem - accuracy isn't the point of the applet, providing useless toys to keep the punters happy is far more important. A random number generator is about as much use as the current memory control panel applet.

Anyway, as for where your 80+MB is - I'd hazard a guess that's taken up by the operating system and associated applications (media player, notes, games etc.). As for any subsequent discrepancy that will increase over time, see the bug I've referenced above for an understanding of why you'll never find out where you storage memory has disappeared to.

PS. Using up masses of storage in the home directory is a sure fire way to run out of space leaving the control panel applet entirely ignorant of what you've done... I wrote a simple shell script which lists the size of each file and directory below the home directory, sorted by ascending size which I find useful to run every now and again to catch any runaway processes. Once upon a time, a bug in the RSS applet caused 151MB of storage memory to be eaten in the ~/.osso_rss_feed_reader directory resulting in the root filesystem reaching 100% capacity (ie. no free space). This RSS directory is off limits to the control panel memory applet which remained convinced I still had over 100MB free space - the memory applet is a toy!