You're just looking at base salaries-

Senior Software Engineers earn over 200k after bonus and stock: http://www.glassdoor.com/Salary/Google-Senior-Software-Engin...

Staff Software Engineers earn over 300k after bonus and stock: http://www.glassdoor.com/Salary/Google-Staff-Software-Engine...

This shows that while Google has "standard" base salaries, employees make a lot more through bonus and stock.

there are 4400 software engineers, but only 285 senior software engineers that shared their salaries. Not sure if that's the actual ratio, but I'm sure it's not too far off.