How drop rate calculated

Beautynbtch - Heavens Tear

I want to ask how drop rate is calculated. Because many ppl say when our lvl lesser than mobs drop rate will be more. But when i try grinding from the sapphire planeshark don with my brother seeker(lvl28) i got drops very rare. But when i try grinding with my wizzie(lvl60) from the lvl55 mobs i got alot drops. I do both of this grind b4 2x drops event start. When 2x drops event start i grinding from with my wizzie(lvl61) from lvl 60 mobs and get rare drops(not as rare as i grind with my bro seeker). Any can explain how this can be happen ?

Innamorata - Harshlands

http://www.pwdatabase.com/pwi/drop

Vitenka - Dreamweaver

The higher you are than the mob, the less it will drop.
And every mob has a different drop rate anyway - in general, higher level mobs drop less often (but the stuff they drop is more valuable, so it evens out)