She was half human and half Betazoid, her empathic abilities coming from the non-human half. She only managed full telepathy in rare circumstances, like with a

She was half human and half Betazoid, her empathic abilities coming from the non-human half. She only managed full telepathy in rare circumstances, like with a lover (Riker) or her mother (a full Betazoid). GP is talking about characters who were 100% human - there was a second one in that same TOS episode, and they mentioned a standardized test all members of Starfleet were given to determine how strong their natural ESP abilities are.