Many times nucs are first year queens, first year hives, and they could possibly be dealing with less mites, disease, and other issues, like an old queen, etc.
While I hear some suggest that it was the overwintered nuc that performed best, I think it really comes down to hearing about EVERY hive, from EVERY Beekeeper. So sometimes we just clue in on some comments more than others. Have you ever heard a beekeeper suggest it was the largest and most productive hive that crashed first? Then on the other hand have another beekeeper suggest it was the other way around for him?
So in the end, we hear of every situation, good bad or indifferent. Many times based on casual observations. If one beekeeper puts on some item like a burlap sack and has their hive come through winter they claim the burlap bag was the reason for success, because the year before they lost a hive that had no burlap bag. And they will claim for the next ten years that a burlap bag makes all the difference in the world. Never realizing that it made little difference. I could pee on one hive and have it come through winter, and claim what? Because I didn't pee on the hive last year and it died, that my pee is something special?
I've had nucs come through winter and they blew other full size away in production. And I have had full size hive come through and blow many nucs out of the water. There are probably many reasons for this, and they were not due to the size of the box they overwintered.
By the way, many times, comments like "nucs being better", is about the same as some casual comments about swarming and the concept that this will "kill all honey production". And it only takes a few repeating it to make it grow legs and become urban legend.
My own observations is that any suggestions that a hive swarms and honey production will be nothing, (like mentioned in the previous post) is simply not true for me. In fact, within ten days (and many beekeepers don't even know their hives swarmed), the bee population is just as strong as they were prior to swarming, honey production is just as great as the swarming hive went through a period with no eggs being laid, and now packs away the honey since they are using far less resources for feeding brood.
But the "If your hive swarms...you will get nothing" will be repeated many times. And beekeepers will panic about that awful thing called swarming. They will split their hives right before the flow minimizing their own honey production, and will do everything in their power to NOT have strong hives for fear of swarming. They will then blame themselves when it happens.