You missed the part where the two SD sets in question differ by 2 moves, the item, the choice of nature, and the EV spread. Generally it's "amount of moves and ev spread that differ" before there's a difference in sets.
In the end it'll always be subjective, but the Sceptile example isn't very good. It's not playstyle, it's similarity of movesets. You can't just say "they both sweep with SD, they must be the same set". However, you have to consider that the differences between the sets are minute on a Pokemon like Alakazam, and significant in the case of the two Sceptile sets in question.