When you create the seek just click on one of the values to change it. It's not really obvious that you can do that.
It is a preselected set of values but you can customize it somewhat. Doing so can potentially increase how long it takes your seek to be matched.
My default setting to start a new online game is permanentaly set to always seek players of -300 to +300 of my rating. For example, right now I'm rated at 1520, and the setting is from 1220 to 1820. How can I set and save it to a different automatic range?