[arien]

The ranges are wide to give room for salary negotiations at the beginning of employment, but also to allow for salary raises for people who are not yet considered to be at a higher level. E.g. you could be a junior developer, but a very good junior, who deserves a pay rise because of performance, engagement or whatever, but still a junior technically.

If you get quite far into your own salary range but never get promoted (if that's how the company works), it's time for some reflection on the situation, for both yourself and the company. Note that the ranges can also overlap between job levels, in some cases.

P.S. Not talking about Google specifically.

[jorgemf]

How do you move someone from junior to the next level? For me if someone has a better performance he/she should move to the next level regardless of the time he/she has been a junior. Are seniors consider that because of their performance?

[arien]

Each company has its own guidelines. A junior could be for example someone who focuses too much in the particular case of a problem rather than looking at the big picture. In interviews, for example, if they present you with a problem to solve and you don't have any follow up questions, you appear as less senior in the eyes of the interviewer.

This is just one of the criteria used, there are more.