Gene, that's a possibility. The amount of liquid in a pre-filled carto varies from device to device. Course, it's gonna have a lot to do with how much you vape, but NJOY may just have a bigger capacity. Course, could have just been that one carto so try another and see how it goes.
Don't throw the cartos out! Down the road you can get a bottle of e-liquid and top them off by just dropping a few drops into the carto. Saves a ton of money! Also later on you'll look at some bigger cartos, blanks, and some other carto systems that have a higher juice capacity.
Sounds like you're on the right track. Glad to hear it's going well and hope you continue to enjoy it!