Well, I think the main difference is this, 'Incrediball' has the genetic potential to have a larger flower head than 'Annabelle', but that doesn't mean that any given that will always be true, it depends on so many factors. Long run, if you planted one of each side by side and gave them identical conditions, you should find it to be true.

Could they be mislabeled. It is certainly possible, like the sea of Kwanzan cherries at my local Home Depot this year that were marked "Weeping Cherry -- Prunus subhirtella pendula" -- nothing very weeping about their upright growth, I wonder how many people bought them thinking they would eventually weep?

Personal opinion: they switched the labels.

Growers are mainly responsible for this. They are simply contractors, and some are better at keeping things straight than others. Sometimes they label things before they bloom, at which point there is no discernible different between two plants.

Some even genericize their stuff. Clinton Nurseries of CT shipped out PG Trees to Home Depots, but they appear to be mostly Phantom. As cool as it may be, people whose trees are getting 12x18" blooms aren't really experiencing Grandiflora.

Forever & Ever's growers have been the worst at labeling their stuff. Inexperienced eyes cannot ID most mistakes, especially when plants aren't in bloom.

Always look for slip-ins and mutations though. You might have something that is rare or different than anything else out there. Right now, my Mariesii Variegata has put out a small stem and leaf set that is completely white - what if it is reverting the opposite direction than "back to green"? Could you imagine an all-white hydrangea from bloom to crown??

Proven winners is fairly reliable so I'd bet that one is marked correctly.

Thank you both for the responses. I have seen occasional mislabeled plants and I have limited knowledge. I can only imagine how many there really are that I don't see. I bought 6 Limelights but I think 1 is actually plain PeeGee. I also bought 2 Pinky Winky and the bloom on them are more fluffy like Strawberry Vanilla instead of pointy/cone shaped so I think those are wrong too. And earlier I had a thread bc my 8 Quickfires I bought didn't all have red stems. Lol. It's tough bc I want to buy the plants when they are fresh in which means before they bloom. I did see the Phantoms at Home Depot and got that too. Many were labeled wrong there though. I think I will plant these close together and see what happens in the future.
Do nurseries/growers give their stock supplements that make them grow bigger in the pot? It seems like so many in the stores are on steroids. Lol. Had me wondering if there was some kind of trick going on here.