Polar vs Nike vs Spark

Sloth_TurtleGirl

Lately, I've tried to use my Polar more. But, I feel like I'm being ripped off on the calorie department. For example, today I walked 1.19 (I was testing). I wore my Polar and used Nike. According to Nike I burned 153 calories. According to Polar I burned 60. I put my miles and time on the Spark People fitness map and tracking and once again came up in the 150's. I really love the idea of using Polar to get calorie totals when I do exercise DVDs. I was just testing it on the walking because I felt my calories were a little low for all the stuff I did yesterday.

Does anyone have any experience with this to know if the Polar is really the correct reading or if I may be doing something wrong?