That's as a percentage varied from the base clock, for people who like it the other way as follows.
Net increase from the base clock....

AMD K6-2 400MHz --->500MHz = 25%inc
Intel Celeron 533MHz --->?733MHz = 37.5%inc
Intel Celeron 800MHz --->?1GHz = 25%inc
Intel Pentium 4 1.8GHz --->2.4GHz = 25%inc
Intel Pentium 4 3.0GHz --->3.6GHz = 20%inc
Intel Core 2 Duo E6300 1.86GHz --->2.86GHz = 53%inc
Intel Pentium E5400 2.7GHz --->4.0GHz = 48%inc
Intel Core 2 Quad Q6600 2.4GHz --->3.4GHz = 41%inc
Intel Core i5-760 2.8GHz --->4.3GHz = 53.5%inc

I don't use that method, but I swear this has come up before..... it's the whole % argument. More than one way of doing it, just depends on what the percentage is based on. Moving right along..... ;P