I would suggest the reason they have chosen to not use an internet carrier at the exact same position is
1) An internet carrier may not work as well to jam as it may be lower power or wider.
2) An internet carrier may get interference from the Saorsat signal, which may degrade the internet performance. In theory this should not happen, but in practice the spots are not really as clearly defined. For highest speed internet ideally you would try not to re-use the frequencies again nearby if at all possible.
3) With a non internet jamming signal they can make it narrow, and adjust the relative power of each to try and just cover the area needed.
Below is the spectrum as picked up in Manchester:
The only one we are interested in is purple 20.185 Saorsat, however it can be seen all 4 spots have 2 internet carriers (wide ones) while the purple also has one narrow one, which was measured as a DVB-S blank carrier using specialised equipment.
Above as picked up near London
and finally as picked up in Dublin. The narrowness of the signal is clearest in the more expensive test equipment. It does seem as if some of the french spots have 4 carriers (to have more capacity) while the Irish and east Scottish beam only has two internet carriers at present. Even on the french system there seems a suspicious gap between the two carriers like one was meant to jam Saorsat.
I would suspect in general they try to seperate internet carriers frequencies slightly, much in the same way as analogue TV transmitters that may be on the same frequency are normally adjusted slightly higher or lower than other transmitters to try reduce interference. Also circular polarisation even does not block the opposite polarisation completely so the blue beam will effect the purple beam too.