Age mostly - and one was made in G Britain.
Singer made small modifications to attachments over time, as styles and fabric changed.
One neat item - Singer included quite different sets of attachments with machines normally sold to other than US markets. Some attachments are rare and valuable in the US that are quite common in other countries.
BTW - Singer did make some hemming feet with wider hems - but I believe those are stamped with the width on them, and they are quite rare. Most of what you see was what was included with the basic set for that era, but a basic hemmer foot included with a 1900 machine is not the same foot as the basic hemmer foot included with a 1940, and things really changed after the influx of outsourcing after WW2. And the improvements in machining tools.
It amazes me at some of what was produced when the work was done without the aid of computer driven anything.