The rationale is simple: form is temporary, class is permanent. Ronaldo scored 42 goals in 07/08. That was an incredible feat for him but it was achieved on the back of class plus form. In 08/09 he still ended up being United's top scorer and 2nd in the league but got nowhere near the previous year's haul. The class was the same but the form wasn't as good. Likewise, Liverpool's end of season run was title-winning form but it will be hard to replicate that. Their class (assuming the central midfielders stay) will be the same, but I can't see them sustaining the level of form over the whole season. Thing is, if you try and analyse why they did so well, all of the same factors are there from the previous run of not so good form which would lead to the conclusion that a good chunk of that good form was temporary.
Liverpool's present first team has a solid keeper, a few decent central defenders in Skrtel, Carragher and Agger a good right back (now) in Johnson and if Alonso and Mascherano stay a top quality central midfield, Torres, Gerrard and two from Riera, Benayoun, Kuyt etc. and all you are short is a decent left back and Aurelio's not even that bad. But the squad, though large, is pretty poor and I haven't seen anything from the likes of Ngog, El Zhar & co. to suggest they're going to give Rafa too many good rotating options.