Construction grade timber is often (actually almost always) dimensioned when it is very green. Therefore unless you're buying something like CLS (which is for studwork so all dimensions MUST be the same) the dimensions of the timber will change as it dries out / seasons.
TLDR it probably was 100mm when it was dimensioned but has dried out.
Construction grade timber is often (actually almost always) dimensioned when it is very green. Therefore unless you're buying something like CLS (which is for studwork so all dimensions MUST be the same) the dimensions of the timber will change as it dries out / seasons.
TLDR it probably was 100mm when it was dimensioned but has dried out.