I also do not understand how "recently added" is sorting photos at all!
I did some testing:
Empty iPad Air and empty iPhone 5s (both iOS 8 GM of course!), "recently added" is totally empty on both devices.
iCloud photostream is turned on on both devices but iPad stayed at home in Flight-Mode while I was at work, so iPad hat no connection what so ever.
So at work, I took 3 pictures (at 6:00am, 6:03am and 6:20am).
When I was back home, I took another picture (4:30pm).
All 4 pictures are correcty sorted by time they were taken in "recently added" on iPhone.
I then turned on iPad, waited some seconds and my pictures were added to "recently added" on iPad as well
but in a totally messed-up order!

They were sorted like so: (6:03am, 4:30pm, 6:00am and 6:20am).
What the hell, Apple?!?
What happens if I have more pictures during time? It'll be a mess! Sorting will be totally out of place!
When you open the pictures on iPad, the correct date/time-values are displayed, only the sorting is... lets call it weird!
-Cobold