The metals in the good category require a large amount of heat therefore oxidation is an issue.
For carbon steel, low alloy steel, zinc, and nickel the presence of sulfur creates a brittle joint; lower temperatures are used to minimize this problem.
Stainless steel and high alloy steel have a low solderability because the chromium alloying element creates oxides that require aggressive fluxes.
In both of these tests, the soldered pieces undergo an accelerated aging process before being tested for solderability, to take into consideration the time a component was in storage prior to mounting to final assembly.
On the other hand, the wetting balance analysis is a quantitative test that measures the wetting forces between molten solder and the test surface as a function of time.