It could be the power of the unit and the consequential radar emissions are too strong. So by dropping the power they loose the range or definition further out.
Speed cameras mostly work on specific frequency bands. Same as the things that sit over sliding doors to auto open them. But like them their range is short probably for the same reason. Over that limit you probably need a license.
That's all a guess but it's my theory.
So buy in the USA and post over.
Till then I'll try the two chrono technique.
BFTA/NSRA County Coach
CSFTA Chairman/BFTA Rep