When you perform a search, the result list is divided in 2 sections:
1) the result list with all the listings on the left hand side
2) the map on the right hand side.
At the bottom of the page you can move to the next page.
When you move to a next page, the pins will update based on the results of that page as the listings appear on the left side. In other words, only those listings that will show on the result list will display on the map as pins.
You will also notice that the pins are different and it mostly depends on available dates and verification status.
Here is what they mean:
The accommodation is a priori available on your dates.
The accommodation has no availability period on your desired dates but you can still contact this listing.
A home that you added to your favorites is available for your dates.
A home that you added to your favorites is unavailable for your dates.
When the pin turns black, it means that it matches the listing you're hovering over on the result list.