Micron rise up even nitrogen pressure did not drop at all.

I installed the new AC outdoor unit. I had nitrogen leak test and the pressure did not drop at all. But When I shut the vacuum pump off, the micron rised up from 500 to over 1000. After that I retook the nitrogen leak test and waited for an hour to make sure but the pressure still did not drop. What would be the cause of this problem? I would be thankful if any one would response to my question.