A high duty cycle means the vanes are closing (as I am sure you know). During engine operation at low speeds and load, little energy is available from the exhaust to generate boost. Trying to get the maximum energy that IS available, the vanes will close. This accelerates the exhaust gas through the vanes and then across the turbine wheel. At idle, the normal range for VGT duty cycle is 60 to 82%.

That seems like a wide range, but your reading sounds normal. I will have to see what mine does this morning.