Microsoft levels used to be 1-15 or something like that. In 2000, as part of "Comp 2000" update to levels and compensation, they changed the levels for a bunch of reasons (one of which was to make smaller, more frequent promos, which is why the bands now each span multiple levels). Each of existing levels was expanded to two or three separate levels. For example, a former level 10 position became either a level 58 or level 59 position. Employees were then placed in one of the newly created job levels. Managers were afforded some amount of discretion in this placement process.
To prevent confusion ("old 5 or new 5?"), they picked numbers that were clearly different from the old 1,2,3... system.
Also, note that the comp change to reduce stock and increase base happened sometime in 2003, not as part of the comp 2000 changes. That's also when the switch from stock options to awards happened.