Exactly.

If GIRL A saw GIRL B make a mistake, she'd also point the accusation at the individual.

If GIRL A saw BOY B make a mistake, she'd also point the accusation at all boys.

Also, when people do this to your face they're usually either (1) trying to bully you, or (2) flirting or playfully teasing you, so this interaction is more complicated than someone earnestly believing the generalization.

Exactly.

If GIRL A saw GIRL B make a mistake, she'd also point the accusation at the individual.

If GIRL A saw BOY B make a mistake, she'd also point the accusation at all boys.

Also, when people do this to your face they're usually either (1) trying to bully you, or (2) flirting or playfully teasing you, so this interaction is more complicated than simply a legitimate belief in the generalization.