Why did an RMAN restore operation not use all allocated channels?

Recently I did a database restore in a productive environment and it took too much time. In particular in the RMAN procedure I allocated 4 channels of type SBT_TAPE. However, it turned out that only two of them were used during the restore although there were al least 3 different backup pieces. Now I wonder whether the use of the channels only depends on RMAN or also on the media management layer (Networker). And for which reason the remaining free channels are not used in my case?