difference in ovulation day every month?

Riley • 28, Married to my best friend, mommy to a happy little girl of 2, pregnant with #2 due Nov `17
So I heard that your ovulation day can differ from this month to the next...
We're ttc baby #2 and I ovulated on day 15 with out first baby. I obviously didn't ovulate on day 15 with this one as we are already at 10dpo and have not gotten our bfp (first baby I got my bfp at 10dpo)...
My question is this: by how many dats can ovulation differ even if my cycle length stayed the same since for ever, even after having our first baby?