Wow I'm a complete wuss, I leave a full hour between clients hardly ever less although I have very occasionally had just 30 minutes because the client can only make a specific time.
Even with an hour I've found that time can be taken by late or early arrivals, answering phone calls, emails and texts, showering, changing, and over runs.
I'm guessing those on 15 minute breaks are working exactly to the booked time - no overruns?