Ovulation vary?

Just need some advice on ovulation. My periods are 28-35 Been ttc for 9months now and this will be my 2nd try of using ovulation tests. (there the cheepie strip ones) If I read it right I ovulated cd9 last month. Now this month I'm on cd13 and my line looks as if its starting to get a little darker. I'm asking is it normal to ovulate diffrent times or is it supposed to be same each time. Tia