Help! Confusing Cover line?

So I have a couple apps plus Glow that I've been using to track my bbt. First cycle charting so I want to understand why all 3 apps have a different cover line? I just want to know if my temps post (possible) O are ok or too low? I'm 15 dpo and haven't POAS but also AF seems nowhere in site! Any opinions are welcome :)

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