I work in this space (Smart Host - market intelligence for vacation rentals) and this is a common practice. Owners and managers keep their calendars open because they treat a channel like HomeAway/VRBO as a lead source.

So if you inquire on a property that's booked, they will attempt to place you in a similar place within their inventory.

Only ~30% of bookings that happen on HomeAway are actually processed through their payment system. Airbnb handles the payments, so the calendar availability is typically more accurate on Airbnb.