Kinda reliable but you need to make sure you have Display only Character Achievements to others toggled
Otherwise anything you get in your other toons might taint the progress. The same applies when checking an alts Progress
Also sephuz is just a default image. Since my mage already has it but it still shows :p
(Also logout and let armory refresh. Might notice a difference or not. Mains should be the same)
Edit:
There were some changes to 7.1.5 legendary drops, essentially first and second drop faster, anything from there on will drop at the same rate or 'chance' whichever word you prefer
Seeing this data is mostly from 7.1 is largely outdated.