It's also a cultural thing I suppose, as I would personally understand all these expressions having just one as the numerator and the rest below.

It may be related to the fact that "mixed numbers" were never part of my curriculum (Wikipedia¹ says they are more common in non-metric regions). Or it may be just me. In any case, I find this notation ambiguous, so I would not expect a compiler to resolve it correctly.

Edit: also, I would rather write (x + y)/2 if I wanted half the sum, that seems much more logical to me than moving non-integer factors around.

¹ https://en.wikipedia.org/wiki/Fraction#Mixed_numbers