I suspect that the way the coaches are packaged would be a factor.
For example, if the DVT is sold seperately, IRM may end up selling loads of the DVT's, and be left with coaches that don't move.
Whereas if they release two suburban bundles, one with a DVT and two coaches, another with three coaches to complement that, they don't end up with stock sitting around for ages.
They could run twice as many DVT bundles as non-DVT bundles, if they projected that sales would be as such.
I guess only they know themselves what is likely to work!