Curiosity on Plextor Premium and securom 7

Hi guys, as you already know Plextor premium has problems with Securom 7, I found that density wrote by Plextor (at least mine), is very different if compared with original cd, I would like to know why. Is there anyone which can explain the feature of Securom7 protected cd?

Ps: the backup still works on various readers/burners but not everywhere.

Thanks in advamce.