They overbook - they had too few people to provide services to seated customers, yet kept taking new ones.
I had two different people switch out for my simple normal polish manicure, because they were sharing 3 or 4 customers at once, even though I had an appointment. I was rushed and barely considered as a customer, but more importantly almost every one of my nails chipped or peeled off before 24 hours had gone, it was just so rushed.