context is that this is a shirt about swimming. Cap, goggles and suit are all swimming equipment.
The son shaves his legs for less drag in the water, and I guess the daughter isn't into swimming and just doesn't shave her legs.

She's so real for that, shaving is a total pain for like no reason.

edit: the quality is bad so I'ma edit in a transcript

Cap..... $8
Goggles.... $12 Suit..... $60
explaining to parents why their sons shave their legs while their daughters haven't for months.... priceless.