Even if there were consistent application by one airline of its policies, your working time aloft would still be as much affected by other flight time differences - different equipment, different routings, en route holds, etc. And the total time available for working is also subject to all the variations of air travel - gate delays, taxi time, queuing etc., which all also eat into the time that you have to work.

So I find it difficult to believe that your productivity is so critically dependent on that extra 5 or 10 minutes of time on board the aircraft that this is an issue worth complaining about, but I suspect that we are digressing.

At any rate, it sounds like the question that you really want to ask is "What is the rule on airline X?" As I say, different airlines have different rules.