If a double standard is the incorrect application of different standards to similar situations, then what is the fallacy when same standards are incorrectly applied to different situations?

It might fall under the weak analogy - see. For example,

Tina: Most people don't wet the bed. Something is wrong with Bethany.
Gina: Bethany is two.

Tina is comparing "most people" to a two-year-old, which they should clearly not be held to the same standards.