Hi Scott,
As you suspect, there is a deeper issue at work.
3SS indoor APs like AP13X and our new AP22X depend on multipath from the walls, ceilings, floor, objects and even people in order to recover 3 distinct spatial streams (and corresponding 3SS MCS rates).
Outdoors we do not have much if any multipath. So we create multipath using dual polarization antennas. The separate H and V polarities are "distinct" enough that it allows the radio to recover two different streams.
Unfortunately, there are only two such polarizations to work with. You didn't mention which antenna you have, but if it is one of our 3 element panels you really only have 2 polarizations. If you look carefully at the arrows on the back, two of the arrows are at right angles to one another. The third arrow is at 45 degrees to the other two. But if you do the vector math, it turns out that the 45 degree one is just partially H and partially V. There is no "third" polarization.
There are such things as circular polarization, however these don't help us outdoors because the E and M fields created by the circular element just ping pong between H and V.
Bottom line, it's really not possible to achieve 3SS rates outdoors without significant local structures that would create enough indirect multipath for the radio to recover the 3rd stream.
In other words, this is a basic property of radio.
To achieve higher speeds outdoors, you need to increase your channel width to HT40, though this exposes you to more interference potentially.