How slick a case feels to your fingers is a much different thing than how it "feels" to the steel surface inside your die.
The difference is how the surface coating behaves under very high pressure. If the lube can hold the surfaces (microscopically) apart, then the brass and steel slide past each other with relative ease. But, if the lube fails to do that, the surfaces stick under the pressure involved in compressing the case diameter.
Your fingers have no way to produce enough pressure on the case to "feel" how well the lube stands-up to the sizing pressure.
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