I believe that they changed the routes this year. The metric takes you directly up to the rim and then around it and back down to the start. The century takes you around the flat lands for ~35 miles prior to climbing up to the rim so that you have 100 miles before you finish for the day.

I had originally signed up for the century because I wanted to ride the whole rim, but I changed our registration last month to the metric since it DOES include the whole rim now.