That's nearly right.
Voyagers are a wet lease, so they (XC) pay for the guaranteed provision of a certain number of sets per day, fully maintained. If they don't use them, they still pay the full whack.
The HSTs are dry lease, which means they pay the basic rental charge, but are responsible for all maintenance costs themselves. Therefore, if they don't use, they don't have to do maintenance, and they save money. This is why all 5 sets have had one trailer car removed, and with the exception of peak summer weekends (and when there are service disruption problems) they will use HSTs to the bare minimum with only two sets diagrammed. It was even being mooted at one stage this year, that they would not diagram any from December.
You will also find they are likely to be laid up between Christmas and New Year (as with most previous years) - not because there is no demand, but because they can claim that there is no demand and the London based Dft will be fooled into believeing it as no one is at work and trains are only ever used by Commuters.