Fit bit problems

I have had my fit bit for a couple weeks now and I was posting my activity and then letting fit bit log my daily steps as normal. But I haven't been logging my exercises the last couple days and just letting fit bit log all activity and have found that the calories burned change ** for example I walked today and then worked out when I got home it said I burned a total of 303 calories but then I took the bit off and went and layer down and is has dropped the calories burned to 293. ** is this normal or is something I'm doing wrong??? And my hrm stated a little over the 303 for my workout at home