Well, earlier people were saying these ratings should just be compared within SP's catalogue, rather than just as a song in general. I haven't been doing it that way though and I guess Murder isn't either.
Well, I'm not quite sure I would say food is art but it does share a similar subjective quality, so I'll stick with that analogy

I don't think a pizza could be directly compared to, say, a burger or a salad or Mars Bar. They're all different. Now, on any particular occasion, I might REALLY want one of those, and it'd be a 10/10 meal. But not always.
But then your original point would've been that I can't give my pizza a 10/10, because for all I know there could be another topping that could be added which would make it better. So, say one day I added some chilli to the tomato sauce to spice it up and I thought, "hey, that's actually even better than it was before!" But the next day when I was preparing my breakfast pizza, I really wasn't in the mood for something spicy. So I would say they ARE different depending on the occasion/mood.
Course, music isn't food either. And I guess our experience of listening to music is different. Music is my deepest love, I would say. I know most people just enjoy listening to music as a way to pass the time, which is fine. I LOVE listening to music. I love hearing new music. I love talking about music with like-minded people.
Anyway, this is all just my opinion. The way I see music (well, hear it, I suppose). To me, it's the art-form above all others. I can't give a precise score to a song. I can't say Tonight Tonight is 9.3. Generally, I'd just say it's an amazing song. One of hundreds of songs that I think are really just mind blowing. So, for me, they deserve a 10/10. For all I know, if Billy played a trumpet at the end it might sound even better. But he doesn't.