5ghz vs 2.4 getting different range, IS normal.
5ghz, while it usually will NOT run into interfearance compared to 2.4.. the higher the freqency, the shorter the distance it will go.
You would be amazed at WHAT in a house can interfear. The strongest bing portable phones, microwaves, etc, who work on the exact frequencies. But even stuff like pipes, metal studs, etc, can all add up and effect it.
On my old router, our downstairs bathroom, BARELY got one bar.. when it was a good, 15ft from the router, same floor. Why?
Likely, as the two walls between it and the router.. are FULL of copper lines for the runs to the upstairs level, etc 
Datalinks link, for inSSIDER is a good tool. It will at least narrow out what other wireless interfearance may be in the are, in helping you choose a clear channel.