Because the raw ramzswap module is able to free some memory sometimes, but not much, not often, only if the system writes over an already swapped out page. But the amount of memory freed is much lower, compared to ramzswap with swap notifier.

I know, because...
- Ramzez is compcache 0.5.3
- First 0.5 ramzswap module with swap notifier was in compcache 0.5.4, not 0.5.3
- compcache 0.5.3 crashes, unless you use an arm test release posted somewhere in the issue tracker.
- There is a config variable you need to define for the modules to be built with notify.
- compcache 0.5.4 doesn't build for diablo kernel, unless you apply my patches.
- compcache 0.5.4 crashes after adding a second swap file, unless you apply my patches (http://code.google.com/p/compcache/issues/detail?id=80)
- compcache 0.5.4 doesn't have backing swap file, unless you apply my patches (http://code.google.com/p/compcache/issues/detail?id=81)

And, finally, a simple proof:
If the ramzswap module is using notification, cat /proc/ramzswap will show some additional info. Look for "NotifyFree", is it there? No? Then it is not using notification.