I would say it's a form of selection bias. You look at only one sample (Azmodan) and evaluate your magic find drop rate. Let's assume the legendary drop rate is 1 in a 1000, with 300-400 MF it's still below 0.5%. Statistically, you'd have to kill Azmodan more than 200 times to see one legendary. However, there's no guarantee - you could even kill him 500 times without a drop (bad luck), while others get a legendary on first kill (luck). Anyways.
It's like picking on elite pack on your Alkaizer farm route that is always there (i.e., the first elite pack in keep level 2). Create a tally sheet how often this one drops a legendary. Unless you get "lucky" (hi RNG), you would curse this one elite and possibly after some time even say "the first pack never drops a legendary, I'll skip it". It's all about choosing your sample large enough. If you, however, choose your sample larger (all elite packs in act 3, or, what is more accurate for a typical Alk run: all monsters slain by you in a farming run), this is perceived differently.
Humans are really really bad in coping with RNG, luck and all these phenomena :-)
got WD froggy off him yesterday
low damage roll =(
i think he is the only boss who gave me legendary EVER when i am starting to think act bosses never drops legendary. I even started to skip bosses because I get more legendaries from from keywardens (4-5 this week alone!)