As has been mentioned earlier this (may) be a case that crew numbers are stretched due to a number of reasons during the next couple of months, therefore diagrams, as in out and back from Waterloo, portions of stock diagrams, and the associated crew diagrams for those legs, have been pulled out to reduce the numbers of bodies required to run the service.
To pull out individual services so that ALL such trains removed were within recognised quieter periods, there would be a far increased level of crew diagram re-organisation required in so far as far more alterations to PNB times, relieving/relief of other crews and potentially changes to sign on/sign off times, the result of which (given the Union/TOC rostering rules that have to be abided by whatever the issue) would very likely result in very little, if any actual reduction in crew numbers needed. That will be the reality for those planning these temporary savings. If it was easy to remove quieter period trains only, and still make the (crew) savings required to still run the service, this would have been done, believe me.