OK,
Let's use another case to explain this, the B-1A versus the B-1B.
The B-1A has an RCS equivalent* to 10 square meters. whereas the B-1B's RCS is equivalent to less than 1 square meter. Two of the obvious differences between the two, are the removal of the A's dorsal spine, and the redesign of the inlets on the B. And that was a redesign that took place during the early 1980s. Obviously, the science of "stealth" has advanced quite a bit since then.
*It is somewhat incorrect to say "The FXYZ-100 has an RCS of .8 square meters..." The proper terminology is " The FXYZ-100 has an RCS equivalent to XX.X square meters", as the standard unit of measure is based on a flat surface oriented towards the x-mitter. Also, most of these figures are based on frontal aspect RCS, not whole-body RCS. An F-105, for example, may appear to be as big as a house when seen on a radar from a side-aspect, yet head on, it presents a smaller RCS than does a MiG-21!