Air dosent weigh anything. If you left the shop with you bottle weighing 25lbs, then you did get 10 lbs of nitrous.
What can happen if you stop getting liquid nitrousw after 7 lbs, is that your draw tube is most likely not positioned correctly, or maybe your bottle isn't mounted in the correct orientation in the car. Then the liquid nitrous boils, and yoiu draw vapor for the last 3 lbs.
Also, even when the draw tube is set up perfect, you never can really get 10 lbs out, usually 9 or so. So its important to find a fill station that charges by the lb, so if you only need 9, then you only pay for 9.
Hpoe that helps.