I'm agree with you that mula rUdra samhitA consists of 11 SAmagAnas ( based on 6 sAmayonI mantras , 3 from samhitA and 3 are stobha mantras ) but in north India it is tradition to recite some preliminarily mantras and additional mantras with rUdram.( Best parallel example is rudram of shukla yajurveda having 8 chapters except namakam and chamakam other chapters are preliminarily mantras and additional mantras to ganapati , surya , narayan , indra , vishvedewas , shanti devtas )
According to a local samavedi all this 5 preliminaries make a personal eligible to recite rUdra samhitA ( clear from the name paviThra varaga , arishTha varga etc ) and last shAntI sAmams are for purpose of pacification . ( If I'm not wrong gAnAnatva + shanchame , other additional mantras and shAntis are also recited with taitIrIya rudram .Sometimes drUrgA sUktaM , PurUsha sUktaM are also recited ).Or more possibly in south India lnly rUdra samhitA is recited.You may refer to published version of it from chowkhamba publishing house.
Regards
Animesh