Why do I find a grade 4 song harder to sing than a grade 7 song?

I'm taking clical vocal lessons, have been working on Mozart's 'Un moto di gioja' and Giordani's 'Caro Mio Ben' (not sure if I got the correct composer name). Well, I was curious so I checked out the ABRSM website for the syllabus of each grade, and I found out that 'Un moto di gioja' is listed in the grade 7 repertoire, and 'Caro Mio Ben' is listed as grade 4. The weird thing is, although the notes of Caro Mio Ben are simpler and don't go as high (or as low) as those in Un Moto Di Gioja, I find it more difficult to sing properly? Other than the tempo difference (and hence length of phrases), both are in pretty much the same range. It actually started getting so bad that my teacher is leaving it off for now and concentrating on the Mozart plus my other songs, for the sake of my voice. Even she says that Caro Mio Ben is hard to sing. Is it something to do with the kind of voice I have ('fach') or vocal maturity? Obviously I'm a soprano so it's not a matter of wrong vocal range.