It's hard for the RathStrat team to get more than 25/40 into the same AV on the AV weekend.
On any other bonus honor weekend, or during the week, it's not that unusual to get 38/40 into the same game (although 35/40, is more typical).

I think it's just a case of thousands of players all queuing at the same time.
And not having have too good of a chance to have your two groups of five get it at once.

You might try the Premade AV Enabler addon.
Each toon would queue separately, but you could at least see if everyone's queue pops at exactly the same time or not.