It's common procedure in order to achieve quick (30 min or less) turnarounds.
It takes time to scan boarding cards, check passports and other documents (visa's etc), check hand luggage in gauge, charge if necessary etc etc.
Once the last passengers are off from the inbound it should only be a couple of minutes until boarding is ready for the outbound.
The issue is that if boarding through the gate started at the same time the cabin was actually ready, you would never achieve an on time departure.
It's the not very nice process of squeezing through as many passengers before the cabin is ready meaning majority of the checks are done and the gate can close on time and cabin be ready for an on time pushback.
Although if i'm reading this right, 16 passengers? That's a bit unnecessary.