User blog comment:Rupert Giles/Global Navigation January Update/@comment-4660173-20150130201518/@comment-452-20150131132746

I agree.


 * Q: Why not just cite pixels?
 * A: Because citing percentages is a well-known trick to make your numbers look more impressive.

Compare:
 * The number of editors on the wiki this week has increased by 100%!
 * The number of editors on the wiki this week has increased by 1. There was 1 before, and 2 now.

If "We reduced the height by 9 pixels." is accurate and not just an example, then the reason for saying "17%" instead of "9 pixels" is because 17 is bigger than 9, so looks more impressive.

It's marketing 101.