I have two sites. Two different antennas are mounted about 10 feet apart on the roof, difference in total altitude is a couple of feet at max. Signal chains are identical – FlightAware filter, FlightAware blue dongle with LNA and filter, Raspberry Pi 3. Feed lines are roughly the same.
In the last 24 hour period, here are the Reports summaries for both systems:
ADS-B 319874 229263
MLAT 145504 70725
Other 136644 94664
Significant difference between the different antennas – the first one is a commercial gain antenna, and the second is a wideband discone.
Numbers aren’t the only difference – when I look at the birds the two systems see, the commercial antenna sees more birds at longer ranges, and the discone picks up more close-in and on the ground birds (and they both see a lot of the same birds, not surprising).
Add to that local topology – take a look at your two sites on Google Earth. From my location, up on the roof, I have a clear path to the local SJC airport (although Moffet Field, KNUQ, is closer to my place). I could site a system a half mile from my place and it wouldn’t have a clear view of SJC; I imagine the numbers would be quite different. If I could get one up at a friend’s place in the local foothills, I’d have line of sight to SJC, SFO, KNUQ, and OAK – and I’m not sure my numbers would be higher, as I’d have a whole lot more transmission collisions to deal with. I’d also have zippo for signals to the West, as they’d be occluded by those foothills. (Also, my buddy up the hill has a lousy internet connection – they’ve been told the best way to get higher speeds is to have a telecoms executive buy a place near them.)
Large differences in reports from nearby systems aren’t that surprising.