Thanks a lot guys for your answers..
It is indeed because of the inline zero detect feature. Thank you John for your explanation and for having pointed me out to 3parug. I found there a great post describing the issue i'm encountering and why.
Here the url to the post in case someone else will need it too: 3PAR Users Group › View topic - 3par "Reserved User Size" and VMware 5.5.0 "Free" size Diffe