Why would you ask for proof, it's an impression. Most worms-themed game they came out with seems the same to me, just the graphics get pimped a bit from time to time.
It goes by the same principle, but what series of a game doesn't? Battlefield1 is still the same idea as 1942, but it's a great game.
Worms Revolution (2.5d, classes, water) and WMD (vehicles, crafting, etc.) add unique features that are worthy of a stand-alone sequel. Whether they are as good as WA is a different discussion, but I don't think it's fair to say they are all basically the same.
So, out of 10-odd, that's 2 titles adding something significant to the gameplay.
I've never played Battlefield, but I can, for example, relate to Need for Speed or COD. And I feel the same about them. While there's some nice additions to those series, most of the sequels are very similar, even fans are saying this (at least about COD, not sure about NfS).