How multiple family properties are listed on real estate sites?

[John Daniels]

While looking for multi-family properties on various real estate sites, I am seeing that some sites will have a single listing for a multi-family that contains multiple units ie 3 doors 1 roof. But then I look on other sites(different listing) and I see that each unit is listed as a different listing but still listed under multi-family category ie unit A as separate MLS listing as unit B.

Trying to understand why the difference. Is it a trick?