The person telling you doesn't know why, and there is no way for them to directly check, but there are a variety of reasons reservations get canceled, reasons they airline doesn't want to pass on to you. Top of the list is being pumped for an airline employee who needs to be somewhere. Then come premium customers such as government agencies/police/military. Then rich people (premium travelers/club members etc). When such people get a flight at the last minute, they are bumping someone else. That someone isn't told why.
In this case, I don’t think that’s what happened; I was able to re-book the exact same flight all three times. In those cases, I should at least be notified my flight was cancelled, but was not. The third rep suggested it was a KLM/Delta integration issue. This is also only one of many issues I’ve run into over my years of pretty extensive travel.
Regardless, you are right; it could be a white lie.
You’re right though. That said, these systems are crufty. It’s true that they get the job done often, and updating them doesn’t inherently mean that they’re better, but they fail a lot. One recent example; I had a reservation cancelled 24 hours after booking three times, with no explanation why. “Normally there’s a note here explaining... but this time, nothing. This just happens and we don’t know why.”