I can think of no 'general' reason why such an arrangement should be necessary. It must be something that's specific to your system (which makes it very hard to offer a valid answer). The most obvious one to me is that there's insufficient pressure in the system to pump water through all 11 radiators.
Maybe the “ Head “ ( pumps are rated in head, the ability to pump a vertical column of water ) the pump being used isn’t sufficient enough for that amount of radiators. Maybe it’s only just coping with the 10. A larger head pump maybe.
Or as bhg says with a system with TRVs 1 rad must have only normal valves so it acts as a heat sink, when all the other TRVs are off. Not calling for heat.