I just saw a W.Rotom with 404 HP how can that happen?

Question

The max HP for Rotom is 304 but this one had 404. What's happening ?

Answer 1

Either one of these happened.

That was probably a Mew that Transformed into it. The only Pokemon that can Transform and has 404 HP is Mew.

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The Rotom-W you saw was hacked, as Rotom-W can only have 304 max HP.