This question pops up occationally, and has several explanations. The shortest and best is here: Ram problem?
The short of it is that you may in most cases ignore the values reported by the WebUI, since it reports a mix of free mem, and dedicated mem. If you want exact memory values you need to log on to the server with ssh and use the apropriate linux commands: http://www.binarytides.com/linux-command-check-memory-usage/6
Theres a rule of thumb I use for my MineOS installation:
The total sum of allocated memory to all my Minecraft servers should not exceed the amount of physical RAM available on my server.