Somebody is feeding you a line or, more likely, they are confusing matters. This is from the tender spec...
In short, you have a fleet that is either provided to you (directly by TfGM) or via the RV process or that you (as an operator) draft in. That is all part of your tender submission.
If you require additional fleet for whatever reason, then it's at YOUR cost (unless you can demonstrate that the fleet you've been given isn't fit for purpose). I suspect that the case is that fleet (provided by TfGM or via the RV pool) hasn't got the uptime expected. Operator says we need more fleet because of X, TfGM say it isn't their issue because of Y.