Rogues; slow or faster MH/OH?

So, I've not played rogues all that much, but I've been playing WoW since OG WotLK. I rolled a rogue this time around and am speccing Assassin (Combat's Killing Spree seems bugged).

I have always known rogues of any spec to have a slow, hard hitting dagger/sword for MH and a faster OH.

Lately, I have seen a lot of rogues, even been 'corrected' and have also seen as BIS items on particular lists like this one, where the BIS is a slow OH. However, I tested on a dummy, nothing but combo builder/user (mutilate/envenom) with Instant poison on MH and Deadly OH, found that a faster OH does more dps; about 10-15% more.

Why am I seeing so many rogues do the opposite? Where and why is there this new 'meta' and how is it at all viable?

I feel like I'm doubting all my WoW knowledge with this.