Welcome to win7forums,
you see only 12 Mb Free because (roughly):
your RAM currently used = 4,23 Gb
Cached = 12 370 Mb (including currently active + inactive cache)
Available = 12 040 Mb (including cached data that can be discarded but not including cache being in use)
So Free = 16 Gb - 4,23 - almost 12 Gb = 12 Mb. If the system needs a Gb of free space, it'll free some of the cached memory. If you disable Superfetch, cached will be less.
I recommend this article:
Measuring memory usage in Windows 7
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Free – This one is quite simple. This memory has nothing at all in it. It’s not being used and it contains nothing but 0s.
Available – This number includes all physical memory which is immediately available for use by applications. It wholly includes the Free number, but also includes most of the Cached number. Specifically, it includes pages on what is called the “standby list.” These are pages holding cached data which can be discarded, allowing the page to be zeroed and given to an application to use.
Cached – Here things get a little more confusing. This number does not include the Free portion of memory. And yet in the screenshot above you can see that it is larger than the Available area of memory. That’s because Cached includes cache pages on both the “standby list” and what is called the “modified list.” Cache pages on the modified list have been altered in memory. No process has specifically asked for this data to be in memory, it is merely there as a consequence of caching. Therefore it can be written to disk at any time (not to the page file, but to its original file location) and reused. However, since this involves I/O, it is not considered to be “Available” memory.
Total – This is the total amount of physical memory available to Windows.
Total – (Cached + Free) = Physical memory in use (excluding all cache data).
Total – Available = Physical memory in use (including modified cache pages).
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Measuring memory usage in Windows 7 | BrandonLive
See details about your computer's performance using Task Manager
Description of What the Available Bytes in Task Manager Represents