State routes had 3 digit numbers. In 1926 the US highway system was created. Others became state routes. Other than this, there really isn't much of a pattern to route numbers other than some clustering. Many routes were renumbered a few years ago.
Some routes have several spurs with 'A' suffixes, yet others have one 'A', a 'B' etc.
Around 1992 several routes in the southeast were renumbered. In that process Route 51 vanished, becoming part of 101, while the former 101 route to Portsmouth became new route 33. Route 16 was moved to the Spaulding Turnpike to take traffic off of the old route. I've tracked down and copied the following notes about New England highways in New Hampshire from James Sterbenz's web page since the links there are currently scrambled. I'll link directly when this gets fixed.