When you look at the amount of memory used with the top command, you see the percentage of memory pages "touched". Since SAS does I/O via the operating system's file cache, memory pages become "dirty". top reports all actively used and "dirty" pages in their report.
With a reboot, all the "dirty" pages in memory being used by filec cache will be flushed and therefore not reported as "touched" with top.
Please note that "dirty" pages are free to be used by any application that needs active pages.
Also, 100% usage of memory without swapping is fine. You should only be concerned when you start to see lots of swapping/paging to disk along with 100% usage of memory.
Please let me know if you need further clarification on this.
Cheers,
Margaret