Before I wrote the instructions I gave you, I performed those same steps on a server I have where I had not previously setup Photos Library or linked it to the iCloud Photos Library to verify my instructions were correct. While I didn't check the count during the process there was a definite lag as the library was filled first with image placeholders and much more slowly the actual images. it took a bit over two days the new Library to entirely settle and now it reports 4865 images (4728 pictures and 137 videos) — the same as my regular desktop, my iPhone, and iPad and iCloud. I can only speculate on your current difference, that it takes a while for things to settle so you might wait a day or so and check again. If it still has not settled then we will have to dig deeper to find out why.
If you are interested in how much local storage is saved by choosing "Optimized" images the 4728 photos and 137 videos in the Photos Library on my Desktop occupies 25.3GB and the newly created library on my server occupies 21.3 GB optimized for iPhone X the Library and app together occupies only 1.04GB of local storage. I have no way of telling how much space the Library occupies on iCloud, but I guarantee it is a lot more than 25.3GB as those are the full resolution images, not optimized for a given platform.
