Because due to the lack of training of CSR's at Bell, many people would get to 6 Tuners and be told they cannot add another receiver as the limit is 6 tuners. Well that was wrong but because Bell then compounded this by a misprint on a programming guide, many CSR's again gave out this incorrect answer.
The correct answer in 99% of cases it that there is a 6 Receiver limit, regardless of them being Single or Dual Tuner machines.
One are where there is still confusion, is on NEW rental installations. The rules there are that a maximum of 4 Tuners (NOT receivers) can be activated on a NEW Rental account. This has not yet changed since the introduction of the 9241, which it should in my opinion.
Just to add a little more background info, you can after actiavting a NEW Rental account of say 4 Tuners (4 x 4100's) add another 2 receivers (NOT Tuners) to that account, either as Rental or ORS (Out Right Sale) although there would be a charge for the Installation and any extra switch's you require. Extra Charges also for the lines to each additional Tuner. So to add 2 more 4100's it would caost around $250 ($50 each for the 2 lines plus $150 for an 8 way multiswitch).
To get around this charge, for the extra 2 receivers, you just change one to a 6131 (HD), this then makes you elligible for a Free HD Upgrade, where you will get not only the 2 extra receivers installed for free but the existing 4 Tuners will also be Upgraded to receive the HD signals as well. (Yes I know the 4100's are SD but they will receive the 82 degree signals)
I know your question was only 3 words and I seem to have written War & Peace but I hope it helps to clear up why people have this fascination of Tuners Vs Receivers.