Heh, that is wierd. What does top say is using the physical memory at that point?
Swapping seems quite high to me, but that might be because Mint has a high swappiness level set as default, I don’t know what that might be by default. In any case it shouldn’t really be a noticeable issue for you. If you really don’t like it try reducing your swappiness, though the trade of is that if you go too far you will get a kernel panic when you run out of memory.
free (as far as I understand it) is reporting memory actually in use there, so it would suggest to me that:
a) after you quit LE, it still takes a little time to actually shut itself down and drain the memory process
b) there is another thing using up the memory that is currently running that may or may not be related to the LE process (but not the LE process itself)
you can get a more fine grained info on memory usage with cat /proc/meminfo (iirc)
you can also use top and sort by physical and virtual memory usage, which will give you the exact process that is using the memory