City have rotation options up front in Aguero/Jesus/Sane/Mahrez/Sterling/Bernardo
Liverpool have Sturridge/Salah/Mane/Solanke/Firmino/Shaqiri
When City rest their forward 3, there's quality to replace them.
When Liverpool rest their forward 3, there's a obvious drop in quality.
Liverpool play Mane, Salah and Firmino so often and in such intensity that it seems almost unsustainable to expect this to remain maximally effective for the entire season. Even if injuries are avoided, the fatigue will eventually creep in and Liverpool will drop points.
Most important, on average a winning PL team loses 2-4 games per season. That Liverpool haven't lose a single game is incredible, but to expect that to continue isn't realistic. Yes, it's been done, but it's anomalous, you can't anticipate for it. Eventually City will retain momentum and Pool will drop points and things will level up. Also, not forgetting that Pool have Arsenal today and City in only a few days. They're not going to Emptihad and beating City days after a tough game against Emery's side.