HRM

Hi, I have been going to the gym for quite a while now and used the machines, I haven't inputted my weight on them just done the quick start and been working my calories from that. I have now bought a HRM which calculates the calories as well. I used this tonight and it says I have burned off more calories on The watch then the machines, hope that makes sense. So what do I go with, what the machines say or my HRM