There are a few things at play:

- Some of them are the same icon but the slightly more modern counterpart (ie where more colours were supported

- Some of them are the same icon but in different states (eg empty recycle bin vs non-empty recycle bin)

- Some are actually intended to be used as crude animations (eg the 4 icons of magnifying glass over the computer are actually just 4 frames of an animation)

- Plus pixels were "bigger" back then so you had to do more with less. But thankfully subtle changes were also more noticeable (again, because pixels were "bigger")