While GTK is a part of GNOME Platform, it also supports other platforms and thus a greater care and stability is required. See this topic discussing the relationship between the two.
GNOME platform design is still evolving. It is much easier and better for stability if the evolution is done in applications than in the low-level libraries. Especially with the different development cycles – GTK 3 is basically finished from a design point of view. Unfortunately, the consistency will suffer in the short term.