Because gender matters even in dog clothes.
Boys’ belly can not be covered totally as they need to be able to pee without taking off their clothing. Small dog clothes for boys are crafted having in mind different breed different “pipi” position.
Girls’ belly can be covered totally and it is important to cover as usually girls have more hair there. And underbely hair collect most of the dirt and humidity. Small dog clothes for girls patterns are made having in mind each breed various body shape and different step length.