Mongoose was designed before the Swipe target limitation was removed. It was designed to be a PPM based on the average tanks weapon (or paw =p) swings per minute. There was no such thing as mindlessly AE pulling entire instances (except Hyjal).
The bear tank in my guild notices that on AE pulls Mongoose is constantly up and refreshes itself well before the timer expires.
I think that is where you get the larger numbers of "average agility' for Mongoose uptime. On single target fights, I'm sure it jprocs more like you would expect it to.
Failure is not an option. It comes bundled with the software.