Photos Import Request but iCloud Photos Switched On

Hi,

I have iCloud Photos enabled on my mac and phone, everything is syncing fine and the number of photos/videos shown on both devices matches (around 16,000 in total).

However, when I connect my phone to the Mac, the import dialogue opens and it asks if I want to import 750 photos, it also states that 2,500 have already been imported. The majority of the photos are a few years old and most of them are photos I imported to Photos directly, i.e. they were not taken with the phone.

Its not a big issue, but curious as to why the Mac is 'seeing' some photos as being new when whey are already sync'd in iCloud.

Thanks