I think, even back when Season 1 was released, we were pretty surprised that it got a 15.I believe their stance has probably been tweaked in recent years. For instance, sexual violence content that would have gotten a 12 or 15 have been upgraded to 15 or 18 depending on the tone or context. For DxD, I did find it interesting that it got away with a 15 but given the age of the characters maybe they decided it would be best to slot it as an 18.
But again, it probably depends on who is rating it. Maybe the person who gave it a 15 felt it wasn't that big of a deal, while the person who gave it an 18 felt it would be a bit much for the 17 year old's mindset (they probably couldn't decide between Rias and Akeno so they used an 18 rating as an excuse to avoid making their decision on who is best girl).
Imagine if they re-released Strike Witches! Season 1 was originally rated a 12, so that'd shoot up.