File count in an iCloud.com Photos downloaded zip

Helping a friend with photos on iCloud.com. She setup a newer M1 MacBook Air after having the same old Mac since 2008 (last running OS X El Cap 10.11). Data from the old machine was manually migrated to the new one. She's not much of a user of iCloud and it is somewhat upsetting how Apple simply sets new machines up to send data (photo, documents, etc) by default to iCloud, and further defaults to Optimize Mac Storage. Seems unfortunate for users not wanting to use iCloud. Luckily my friend keeps her own backups.

Out of curiosity we did go on iCloud.com to download what went up there after photos in iCloud was switched off (especially due to this Optimize Mac Storage setting). There were just over 2000 photos so we downloaded 1000 items, 1000 items, then the remaining items in three separate zips (due to the 1000 item download limitation). When the first zip was opened there was less than 950 items inside, the second zip had 1020 items, and the last had what was expected.

Why would selecting 1000 items in iCloud.com Photos, and downloading, create a zip with less than 950 items? and another with more than 1000 items? Also not allowing a complete Photos download (ie. the 1000 item limit) had some identical names between zips, as such all the files can't go in the same folder. The iCloud.com Photos download did not inspire much confidence to get back the original items.

Assuming there were no new photos since moving to the new machine we'll likely just use the backups.