Most editors set their per page rates based on the amount of time it takes (on average) to edit. So they are indirectly charging based on an hourly rate.
So for example if someone charges $5/a page for a copy edit, he may expect to edit an average of seven pages an hour - and hence make $35/hour. He may be able to work at slightly quicker clip - say nine pages or 45/hour or a slower - one 5 pages at @25 an hour -but overall editors are looking at how much money they will bring in over the amount of time it takes to complete the edit. $25 by the way is on the low end of the editing scale. It's not uncommon to find editors who charge twice that or even more.
It may be a better marketing move to charge by the page, but it's not necessarily a better deal for the writer. In most cases the hourly vs. page rate would come out to the same fee and in some circumstances, a writer can actually make out better on an hourly rate.
The difference between Gilloughly's per page rate of $75 or 80 vs. an hourly rate for the same work of $350 - is quite broad. One way to avoid underbidding on a page or flat rate is, if possible, to eyeball the first few pages of a manuscript before giving a quote. There's nothing wrong with customizing your page rate to reflect the scope of work of a particular job. If you see that there are serious grammatical errors and it will take you a few extra minutes to edit each page, then adjust your per page rate accordingly, so that you are compensated fairly for your time. I once edited a novel written by someone who was not a native English speaker for a flat rate - it was a decent flat rate, but i took a huge bath on the project - almost every paragraph needed work. I was averaging 1 1/2 to two pages an hour when I had budgeted my time for 4 or 5 that. He spoke English very well, so I wasn't expecting the grammar and syntax to be as problematic as it was.
As a professional freelancer you should always have an idea of both how much money any given job will bring in AND how much time it will take to do the job.