I’m having problem to find why fonts are bold, even tho i didnt apply bold, but half are not bold. which is kind of more confusing.
so some are bold and some are not.
[Warning, link plays arabic vocals !] 🙂
check following screenshot and here is live demo link (on demo link will play audio, please click on pause and it wont play again on refresh)

if you see on top right corner, that arabic word is showing normal and next to it the other arabic fonts are bold.
on second line, same second word is not bold, which it was on first line. o.0
One possibility (that I have experienced) is that the font-face you are using does not have bold for the things you are looking for. (btw, I can’t tell differences in my browser, Opera, everything looks gray).
I experienced this with a font I implemented using @font-face and font that the font I had chosen didn’t have any bold versions at all for to use.