If you check with a debugger you will most probably see that (up to the available number of logical cores) the number of threads spawned is equal to what you specified as an argument to EnableImplicitMT. What’s happening is that some of the threads are idle as they don’t have any task to pick up. The smallest granularity of a multi-thread task in TTreeProcessorMT is that of a single TTree cluster (tree->Print("clusters") gives information about the number of clusters in a tree.
Cheers,
Enrico