BBT Charting, please help!

I've been charting my bbt for 4 months now, and have noticed my luteal phase is typically between 12-13 days, except one month it was 16. Even though there is a temp shift, is it possible that I actually ovulated later on the cycle when my luteal phase was so long? I know they say your luteal phase typically doesn't change from cycle to cycle, so I'm wondering if there's a chance that the small temp shift wasn't actually my ovulation date until later on. Thank you in advance for your responses, the support on this app is amazing!