When I compare my Elementalism Focus with my Hammer and Shotgun, the tooltip claims I would lose 21 critical rating although all three show the same 1156 critical rating from the glyph.
If I equip the focus and compare the shotgun, it shows no difference between hammer and shotgun as expected, but the same +21 critical rating for the shotgun compared to the focus.
(No, I don’t have any addons that fiddle with the tooltips)
The only thing I can think of is “Does the character have all of the weapons’ levels maxxed out?”
Of course 
Also, judging from how weapon expertise is presented, it seems to be flat crtical chance and has nothing to do with rating.
The tooltip probem itself cleared itself today, right after I leveled the focus to 20. Now, the comparison only shows the difference in weapon power as expected. ¯\(ツ)/¯
Edit : well, it’s not completely gone
683 != 611
