Please also note that although the antennas are omnidirectional, if you mount the E400 on the ceiling, you'll get better coverage underneath the AP than you will above the AP.
since the e400 is designed for ceiling/wall/desk-top mounting the desired radiation pattern is down/out/up for the various mounting options so the antenna radiation patterns are designed to be semi-directional. We do not want to "waste" energy up into the ceiling for ceiling mount so the antennas are designed so they radiate more thru the front face of the e400.
A theoretical omni-directional antenna radiates equally in all directions, but when implementing a real-world antenna there are many factors that make this impossible (such as the physical construction of a metal radiating element with a connector and plastic radome, an slightly imperfect VSWR will result as well). In addition, since the radiating element is in the proximity to other metal contained in the access point printed-circuit-board-assembly and the enclosure plastic the pattern will be modified. Hopefully this helps explain things a bit.
How do you account for the overall patterns of the two antennas for each band? Since the e400 has two 2.4GHz antennas and two 5GHz antennas the composite of the two should be considered the coverage pattern since they always operate simultaneously. For a ceiling mount AP you want as much of your energy to radaite towards the client devices and not up thru the ceiling or wall.