I'm with Mudpie and believe that the bike should be able to shift into any possible gear combination. While riding crossed over is less than ideal, causing increased friction and wear, there are times where it can be handy such as to climb a short rise where you don't need or want to shift the front.

As others have noted it might be that you need more cable tension, but an easy test for this is to try to make the shift by pulling the bare wire away from the frame at the down tube. If it shifts better this way, add a bit of cable tension. If it shifts the same, your outer limit may be a hair tight, and should be backed off (slightly) to let the cage move over a hair more.