Personally, from my experience, I think the inconsistency has to do with the flight loads. 1st time out of JFK, coach went out with most of the middle seats open. Very light flight load. I had no problem with my bag then, neither did all the people who bought a crap load of stuff from duty free + laptop bag + wheeled bag.
Out of DXB, still light load but not quite as light. People had to check their second bag or the wheeled bag if it was too big.
My last flight out of JFK was a nightmare. All around, not just baggage. We just moved to DXB, so I had all my jewelry, marriage and financial documents, diplomas, etc in my carry on bag. The flight was oversold. They stopped everyone in coach, measuring the bags, weighing the bags. I ended up in tears because they were belligerent, insisting I couldn't take this wheeled bag on (its smaller than a 22'', maybe like a 20"). In the end, the ground crew left the decision up to the porter who took my bag and placed it with his in the cabin crew area. But there were some other people who were not as lucky. It was a mess.