They are all wrong and the actual flex is a 2...
Seriously though, those numbers mean shit and the .5 to 1 difference won't be noticeable. As long as you purchased the correct size for the kind of riding you want to do and understand the way the manufacturer wanted the board to be ridden you won't have a problem. Riders in general need to realize that flex ratings are probably the most inaccurate spec given to a board. Rather than reading the number look at features of the board, is it a wide version? if yes then it probably is a bit stiffer, what size is it? bigger normally will mean its stiffer, What's the core made of? Does it have features like poprods? Stuff like that is really going to give a rider a much better idea of how a board is going to perform vs some stupid number that really is just going to give you a rough estimate of whether the board is stiff or not (Ex. a 1 rating vs like a 8).
As for the difference i would trust whats on your board vs the website as a website like Lib Techs is going to be constantly updated and something like that could have easily slipped through the cracks.
That's an interesting way of looking at it. I would have thought that a snowboard is a piece of precision sports equipment and if they were going to make it "wider" but keep the same flex pattern, they would like thin out the core by a small margin vs. just "cut different sizes out of the same blank" as you would be suggesting.