It is not all hardware. The construct of the building makes a big difference as well. If there are firebreaks constructed of cinder or steel that will greatly attenuate signal as well. 5GHz has at least half of the wavelength as 2.4 so the distance is a great factor as well.
Also RF is based upon ground wave propagation, meaning that it follows the lay of the land, so being at a higher level doesn't guarantee better coverage, it can actually hurt coverage as depending on the strength, one could be overshooting the target. You may find the depending on the height of the router the signal may not drop in the and be putting you on the outside of its useable range.
Like was pointed out the only real separations in channel are 1, 6, and 11. Anything in between is a mixing of the outlying channels, so the chance of overlay interference exits.
Like I said before too many factors to tell for sure. Does the Signal get better when closer to the router, that points to some sort of interference, be it rf or physical barriers/ distance?