Why? The employer doesn't look at her skill set. Sarah has great customer service skills and knows how to handle money and stock - she's perfect. However the employer looked at her job title and decided a barmaid couldn't possibly be suitable based on simple heuristics they probably weren't even conscious of.
Sound familiar? Over the years I have had numerous job-titles that reflect only a percentage of the job I do. Company X will think a Digital Producer does one thing, Company Y another thing and Company Z has never heard of Producers but calls them Project Managers and Company W calls them Account Managers. There will always be an overlap of skills and sometimes new skills must be learned.
The reality is the label means very little. I've been involved in strategy, wireframing, site mapping, user testing, requirements gathering, content mapping, card sorts, working with clients, creating project plans, executing project plans, research, managing budgets, prototyping, invoicing, proofing, copy writing, writing test plans, testing, writing scope docs, writing service level agreements, recruiting perm and freelance staff, content management, studio manager, traffic managing, writing tech specs, events management, etc.
All to varying degrees of course - the point is while I might excel at some of these things and be a novice at others the list of skills can't be encapsulated in a short job title.
I believe it's important for employers to look at a person as a whole and not pigeon hole them based on a few labels. At the same time I believe it's important for employees to be careful of becoming a jack of all trades.
Everyone can learn and evolve in their career, most won't become a master of everything. There will always be skills you're a novice at and others where you're an expert and many more sitting in the middle.