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It could be a bad gauge I guess.
When a HPA tank is filled the air inside it is warm, so as it cools it is natural for the pressure to drop. If the tank is cooled, the pressure will drop more. When you took it out of the car and were using it, I'm going to assume it was absorbing heat from the air and maybe sunlight. That would have brought the pressure back up.
In short, you were right. Nitrogen/HPA changes pressure under different temperatures. It isn't nearly as sensitive as CO2 is however, and your output pressure is controlled by the reg, so it will be consistant where CO2 would not.
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