The tiers and levels were discussed as NOT being put in place as a preventative measure for sand-baggers, but I think you are seeing the inaccurate nature of those claims. No one knows the exact algorithm, but it is very obvious that the new system does not allow lower scored games to affect ones overall average a much as it used to. Good for helping to stop sand-bagging, bt bad for the casual player who ends up progressing too quickly.
WGT doesnt give out the specifics, but they have other things built into the tier progression calculations to help prevent most from moving up before they are ready.