Shoes and boots are made over lasts. Whether the company changes hands or not, lasts wear out. For one reason or the other (complaints, manufacturing issues, etc.), companies often take that as an opportunity to remodel or change the last as they are replaced.

Compounding all that is the fact that despite popular opinion there really is no standard length (stick) or girth for a size 6D, for instance.

Lasts are often designated a particular size in accordance with other, often sociological considerations, that have struck the makers fancy. If a maker is producing shoes for women, for instance, they will often be marked a size or even two smaller than girth and stick would indicate. This caters to the vanity of some individuals who do not like to admit that they have big feet. And the designated last size will then be transferred to the shoe itself in the form of the information on the box or printed on the lining.

Depending on many factor...including country of origin...no two shoes marked 7D will fit the same. Sometimes there will be drastic discrepancies.

Beyond that, feet change. Especially as we get older.

If you rely on shoe salesmen or the size marked on the box to get a fit, you might as well flip a coin.