Originally Posted by HTupolev

It is not. 11->13 and 25->27 are both 2-tooth jumps, but 13/11 is more than twice as wide a ratio as 27/25. Because the number of teeth on cogs must be integer, it's almost impossible for the spacing to be even. For instance, a 3-speed cassette with cogs of 12-18-27 would have even spacing, because each cog is 50% larger than the previous.