Differnt 4600 APs Show Difference GPS Strengths At Same Site

I have four 4600 APs at one site, a water tower at 140 feet AGL.

The GPS two of the APs, the GPS Monitor shows very strong GPS signals from 8 or 9 satellites, one of the APs pointed north and the other good AP pointed West.

One AP shows GPS at Low Sensitivity, not nearly as good as the two mentioned above, with that AP pointed south.

The other AP shows GPS at Very Low Sensitivity, very poor, with that AP pointed east.

Any ideas as to what might be causing the differences in the GPS signals between these different APs, where they’re all at the site, on top of the water tower, at the same AGL and within about 12 to 15 feet of each other?