Is it because the designers of the course sentences were intending to teach us something about gender in grammar?
I mean, the second word in this sentences goes well with Bob's being male, and the 3rd & 4th words comply with eachother (that is, "brown" needs to be used in female form since "skirt" is female).
If so, the lesson is, the gender of the possession hasn't got anything to do with the gender of the owner, or vice versa.
People forget language comes with its culture. And no, it's not the norm for men to wear skirts. We're not talking about wherever it's ok or not. We're talking about the target language and its related culture
A guy can have a skirt for sale in his clothing shop. We often call shops by their owner’s name if the shop is small and we are regular customers. Bob can also be a clothing designer, and we have plenty of those, too.