I've noticed that, too, with my knitting. I'm doing this Dr. Who scarf. This thing is ridiculously long. I think it's supposed to be 15 feet long when done, but I'm not sure. I'm about 3/4 of the way done with it. It's all garter stitch. I think that you relax as you're knitting and your tension gets a little loose. That's why it seems like some garter ridges are farther apart than others. If it bugs me, I rip it out and do it over. Try holding your working yarn a little tighter. You didn't say if you were knitting English or Continental. Wrapping the yarn around one of your fingers one or two times helps.