Such a difference in cals!!!

When I ride the bike it says that I burned 330 cals in 30 min. But, I just bought a HRM watch (it's a sportline) it says that I only burned 105 cals in 30 min. And, when I type it in the website it says I burned 260 cals in 30 min. Now I know you get what you pay for but is it really that off? Which do I believe? There is such a big difference. The watch doesn't have the strap that goes around your chest. It was about $40.00. I just wanted something to wear when I go running outside to give me a clue about what I'm burning.

Any ideas as what to do or believe would be great!