That's true, Laker. The book used to say something about "unusual differences to the scheduled flight time" would be paid but no one would define "unusual" or whatever term they used and of course no one ever got paid extra for their time.
The system works for you occasionally, too. High MN, better tailwinds, large block time and good vectors without holding sometimes (very occasionally) get a good reduction in flight time.
Take what you can, when you can. Give nothing back beyond what they pay for. (That's their mantra para-phrased.)