If your memory is less, there are a few possibilities:

a) your program uses a GUI. X11 might allocate memory for your program which it isn't giving back

b) your program doesn't really exit. Check your system for defunct processes

c) some OS caches might fill up memory. Did you for example access the file system with your script?

d) your OS or some other process interacting with your script might have a memory leak

You might save the output of 'ps -elf' to some file between a few starts of your script and see if you can spot any differences in the memory footprint of the processes. If not, either the differences are too small or your OS is using up the memory.