Talk of "stable" and "current" had me confused at first as well.
As I understand it, stable is a major release (10.0, 10.1 etc) at a point where Patrick is happy that everything works well together. This is then only extended with updated packages where there are security issues.
If a Current installation started from the same point, say 10.1, then it would have been extended with the same security updates as a Stable version, plus other additional packages.
The differences between the ChangeLogs will show you the additional packages in Current.
One could say that a release is when Stable catches up with Current.
I'm not an experienced Slackware user but as I run a Stable installation, I manually download then use kpackager in KDE to remove old and install new packages.