It's a feature! You've paid for that RAM, after all. Linux will do its best to use
all the real memory available for something, whether allocated to programs
or for cache, and to keep things out of swap when possible.
You can check the accuracy of phpsysinfo at the command line with the
command "free", which will break things up a little further:
$ free
total used free shared buffers cached
Mem: 776640 623112 153528 644856 135284 356272
-/+ buffers/cache: 131556 645084
Swap: 265032 1144 263888
That's probably similar in proportion to what you're seeing -- 620MB of
776MB are in use, but 131MB of that is buffers and cache.
Cheers,
--Rich