To discuss why this is so, we look back at IBM OS/2
I don't know if you remember OS/2 but it was an Object Oriented OS. (OOO?)
The concept was beyond OOP in that if you made a change to something it would propagate where needed to the rest of the system.
In Windows however I've found it not to add some user to a share even when you thought they should be there. After awhile you learn to accept this and work around it by other procedures. I noted some already.
It appears that Windows seems to be OK if you set it up, but when you adjust settings or users you get to do more checking. My old example was to share something, then add a new user and demo that the user on the other PC can't get to it till we reboot or unshare, reshare, and maybe more adjustments.
Enabling the Guest account is a work around but I like to avoid that since that opens the door for other problems.
I see Mark has covered the Public Folders issue in 7. This one can confound many.