Hi,
There is an excellent Oracle Document covering this on the Oracle site
Document ID 1663862.1 it give in depth information covering your monitoring issue.
This should at least point you in the right direction.
Due to the way that the MMU and ZFS behave, this can be an issue for monitoring tools.
I've also just noticed that you say that you've recently noticed the increase in memory utlisation, an other possible cause could be memory leakage caused by applications code not fully releasing memory. Do you have any trending information, does your monitoring tool produce such information - this is generally easy to see over a longer term.
Regards
Gull04