Decided to do a little bit of research. Easiest way is to reference my own tweet here with the images, sorry:
The reason they're top is quite simply because City/Liverpool have dropped their standards. With 13 points after 17 games they'd be the following in other years:
19/20 - 13 points off the top in 3rd
18/19 - 9 points off the top in 5th
17/18 - 13 points off the top in 3rd
16/17 - 7 points off the top in 4th.
They've got a long way to go and I genuinely think that if City and/or Liverpool pick up anything even remotely close to their old form they'll stroll ahead of United.