A+ Allowing Overlapping Appointments to be Booked

We have a consultant who booked an appointment for herself at 1:00PM for a 60 minute service. However, on the front-end, A+ was still allowing a visitor to the site to book another appointment at 1:45 PM, overlapping with the first appointment.

I checked the settings, and by changing Boundaries Detection to "Detect Overlap" I was able to have the option to book a conflicting appointment disappear from her front-end availability, but I'm concerned why this was able to even happen in the first place (with "Exact period matching (legacy)" selected). In particular, I'm concerned that other conflicting appointments might have been booked for future appointments without anyone noticing, and we now need to go through and check them all.

What am I mis-understanding about the Boundaries Detection option?