I’m not a hairdresser but worked at reception in a salon to earn money for university (in the USA, if that matters).
I see OP’s point.
Our stylists were precise about timings, and they knew their clientele. Before booking over the phone (before online bookings) I needed to run over to Sue and say it’s Sarah B who needs a highlight & haircut, how long does this take.
Normally the clients were on file and I could look up note cards without having to interrupt the stylist, but occasionally—especially new clients—it could be a 15-30 minute difference.
For anyone saying it’s not a problem, put yourself in the position of the next client and ask yourself you’re happy to wait 30 minutes.
Is it a big problem? No. Does it matter? Yes. Hairstylists are professionals (underrated in the UK, in my experience) and while they might say “no problem, I’ll take you from a long bob to a pixie cut” or “yes, let’s get you back to a warm red with your colour,” it throws the routine and has an impact on their work day.
Of course many stylists are confident enough to say they can’t do it right now, but many others aren’t. Partly because they fear repercussions from an unsatisfied client, but often because they truly want to make you happy and feeling a million when you leave.