If there's one thing you ought to be able to count on from a sequel above all of the rules about escalation, more character development and the exploration of some new ideas: a sequel should always be a sequel. If it's going to claim openly to be a follow-up to whatever the last movie released in a franchise, it should have the decency to actually follow through on that.
But it seems that that simple rule doesn't actually seem to matter to some movies. Despite promising a continuation of established - and usually lucrative (which is often the key point) - franchises, they stand as monuments to Hollywood's occasional insistence on packaging things in a way that best suits the bottom line, rather than the audience.
Sometimes, the transgressions are slightly more forgivable - like when marketing campaigns suggest that a prequel or a remake are a sequel - or when the film-makers are playing with the idea of anthology films, but there are some cynical examples out there that leave a slightly bad taste. And invariably, when a film is seeking to use an established brand to sell itself, you just know it wasn't good enough to survive on its own.
Basically, what you need to know is, if it walks like a sequel and quacks like a sequel, it's not always a sequel...