Changing ovulation days?

Alison • 35 years old, married in 2012 and ttc for 3 years this June. Praying this is my year!
Does anyone else experience this? 
The first time I checked my ovulation day I had a surge on day 17. Now, 3 months later, the last 2 months has been day 11. Is it the ovulation kit brand or can your ovulation day actually fluctuate?