Its all to do with the manufacture of the receiver
Some tuners have only one filter in them (the fiilter works in a similar way to the user as a bandwidth filter on analogue equipment - 18 / 27 MHz etc) to allow the receiver to filter out unwanted stuff
The ceramic filters of commercial or subsidised boxes are limited to the higher symbol rates of 18 - 45000 Ms/S and so cannot be used to receive the SCPC broadcasts - this is why the older receivers (and to some extent BSKYB stuff may not be good at finding everything, even though there are further restrictions in the firmware and OSG)
Newer receivers have a dual circuit of filters that has switching between the old limits and the new of 1 - 22 Ms/S
You will not see the switchover in operation, and there is no point in restricting the upper limit of 45 by the manufacturer as it would limit overall sales
There are some broadcasts up there that do have higher symbol rates than 300000