Inact v. Free Memory

[Micah Bushouse]

I have a quick question regarding the differences between Inactive and 
Free memory.

Context:  I'm running 4.9 stable as a desktop machine.  Right after a 
fresh reboot most of the system memory shows up as Free (using top). 
Then, after a few hours of work... running X, web browsers, and the 
like, most appears in Inact.

My question is...  after I shut down all programs, ctrl alt backspace X, 
and get back to a terminal, why does top still show all the memory just 
freed by my desktop programs as inactive?

Please see my attached top_output.txt.  You'll notice that after adding 
up all the memory in the programs that are still running, the total 
doesn't come close to reaching 308M Inact.

Thanks,
Micah