Did someone mention psychology
It really comes down to perception. In order to perceive someone is out of your league you have to have a very good good perception of how others see yourself to base this argument .
Unless you're looking for a certain criteria say looks or level of wealth, which is different.
Its an ambiguous statement at best, any girl that pulls this shit is seriously insecure.
Fuck with a girls head when she comes out with 'I can do better' line
'no you can't you have small tits, no good looking guys gonna want to be with you when he could do better.