Boeing Service Letter 737–SL–27–069–A explains it all.
Here is the gist of it.
-100 -200 aluminium structure elevators & tabs
Classics have composite elevators & tabs - titanium tab rods are better thermally matched to the composite elevator tab structure - results in less tab trailing edge deflection due to variations in temp. (than an aluminium rod set) so the limits were increased as a result.
The other factor classic to NG is mach trim compensation differs for aerodynamic reasons, reflected in type of FCC.
The -600 -700 -800 tolerances are not all the same as each other in the AMM.
Hope this helps