Understanding photo libraries when migrating from iPhoto to Photo
Some more information about the structure can be found here. The three folders you see all reference to the same set of image files on your Mac, using hard links.
The 150-photos difference MAY be due to one of these reasons:

Your Photos library already had some photos before you migrated iPhoto library (More likely).

You had RAW and JPEG pairs for some photos. iPhoto counted each pair as one photo, but in migration they exported separately, increasing the count (Less likely).


The Photos library should be the superset containing all your photos, but the only confirmatory test I can think of is to extract the file lists from the SQL databases of those three folders and compare them.