Photo creation date changed on last sync

I had been having a problem with photos syncing to my wife's Macbook Pro (running El Capitan). We were working with a senior tech who had us change some settings and the computer then took about 2 days to sync the entire library. Up until that sync was completed, there were a few gaps of missing photos and videos - a month in 2016, a few in 2017 - not full months, but groups of days or weeks that photos were not getting to the Macbook, but were clearly on her phone and in the cloud account.

Now that the sync is done, I noticed the file creation date on some, but not all photos on the Macbook have yesterday's date rather than the date the photo was created. I had to speak with another senior tech today and he told me that was normal behavior and there was nothing that could be done. How is that even possible? If it was normal behavior all the photos would have yesterday's date, not just some of them. In addition, the entire file structure when you view package contents has changed. I have copied over the Master's numerous times to an external drive because we are sorting these photos outside of Photos to do some comparisons and each time, including yesterday afternoon before the final sync completed, the file structure remained constant and the creation date of photos never changed. Now all of a sudden both of these have changed.

If someone could explain how this happened it would be very helpful. Again, if they all changed I would be frustrated but at least it would be consistent. If the modified date changed, even though they weren't really modified, I might get that. But the random changing of only some folders and only some image creation dates makes no sense.