Help with Shareable Transmogs

[Trustlvlfour-blackwing-lair]

Question that needs an Answer

Can someone explain to me how like for example my mage and my priest have some transmogs over lapping and some not. Like i know i have a priest specific pvp mog that shows up on my mage transmog list but bunch of other priest pvp set dont show up on my mage. What determines this and how can i predetermine if a set will show up on my other same armor type character? THANKS!!!