7.8. Free Memory as Reported by free Keeps Shrinking.
The "free" figure printed by free doesn't include memory used as a disk buffer cache—shown in the "buffers" column. If you want to know how much memory is really free add the "buffers" amount to "free." Newer versions of free print an extra line with this info.
The disk buffer cache tends to grow soon after starting Linux up. As you load more programs and use more files, the contents get cached. It will stabilize after a while.