This is a good place to read
http://ismacs.net/singer_sewing_mach...ist/index.html
Click on the link at the top and scroll down and you can read snippets about some of the models - and also see that for the most part, there is not a lot of rhythm or reason behind the number system. Some times there is similarly- the 28 is the 3/4 version of the 27, and the 127/128 are updated versions of the 27/28. But the 99 is the 3/4 size of the 66. The also used 15 for both industrial and domestic machines - And the updates to both kept the same model number and changed the suffix.
I don't think there as any overall plan to Singer's numbers.