I don't think I can give an exact answer to the question, but if I were to hazard a guess, it might be that they pad out certain things in the anime to fit a more episodic structure and, assuming they considered the anime might not get a Season 2, switch some arcs around to have the more climactic arc be at the end to feel more like a complete story. The arc where Masahiro's friends found out about the relationship is technically the last arc of the ones they adapted from the manga chronologically, but the anime actually ends with an anime original arc loosely based on the arc that takes place during Volume 2 and ending with the ring scene. Admittedly though, there are some changes and additions the anime made that don't make sense to me, but that's pretty much my guess.
Okay, that kinda makes sense, so thanksss
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I'm so confused, I came here from the anime to read the rest but why is the manga so different from the anime? I mean, the scenes are similar but the chapters........it's kinda hard to explain since I'm not good at putting it to words but you get my point, hope so....