interesting. It kind of makes sense as you have less downtime between shots. This effect is especially amplified because hunters are somewhat bugged and you're forced to use a 1:1 rotation instead of a 3:2 rotation. I'll replace my Vashj bow with a Don santos hunting rifle next Hyjal and see if the DPS is higher or lower.
Though, I have a feeling you will cuck yourself if you're not aware of your attack speed increases. DST + haste pot+ Rapid/Bloodlust will probably end up with you having a way too quick attack speed and you'll loose DPS in that window compared to a slower bow.
Edit:
I just did some testinge between Don santos hunting rifle (Rank 3, your source) and the Lady Vashj bow (rank 9, your source). I organized the information from my rudimentary test (see picture).
https://imgur.com/a/6Ok3i