The glass transition temperature (Tg), in-plane diffusivity (D), and effective viscosity (ηeff) were measured for the same thin film system of poly(isobutyl methacrylate) supported by silica (PiBMA/SiOx). We found that both the Tg and D were independent of the film thickness (h0), but ηeff decreased with decreasing h0. We envisage the different h0 dependencies to be caused by Tg, D, and ηeff being different functions of the local Tg’s (Tg,i) or viscosities (ηi), which vary with the film depth. By assuming a three-layer model and that Tg(h0) = ⟨Tg,i⟩, D(h0) ∼ kBT/⟨ηi⟩, and ηeff(h0) = h03/3Mtot(ηi), where ⟨...⟩ denotes spatial averaging and Mtot is the mobility of the films, we were able to account for the experimental data. By extending these ideas to the analogous data of polystyrene supported by silica (PS/SiOx), a resolution was found for the long-standing inconsistency regarding the effects of confinement on the dynamics of polymer films.