When I first got my RadarBox, I was running the SBS-1 and RadarBox side by side for quite a while.
I didn't see the strange differences that b744 reports and I can't think of any reason why this should be the case.
b744, when you say that there was a difference of 20 contacts over a 24 hour period, is this based on the logs?
You report that you have no filters selected, remember that you can use height, aircraft, range, airport, airline etc. If you had EGLL selected as the airport in the quick filters and there was no valid route with EGLL in it, then the aircraft would be filtered out on the map, but would appear in the list.
It would seem that there was a fliter in there somewhere - other than that, I don't really have any ideas.
Certainly, from my experience of using both boxes, whilst there is always a slight difference between the 2 displays due to timeout settings and antennas, Radarbox consistantly displayed slightly more aircraft than the SBS-1.