I only have Windows.
I gave up on BBMax a while ago as it showed crazy figures. I read a post not long ago that I think said speedtest.net uses the same engine. So

.
In BBMax's case a while ago it seems it had a single 100Mbps pipe to use. That wouldn't help accuracy.
The only tests I really trust are BT and TBB. The oddities in TBB seem to be specific to certain unknown circumstances for particular users. I always find the two tests very consistent when run in sequence.
Bob: BT Broadband >> Prodigynet >>
Newnet Lite.
Purple Cloud for email, web space and domain.