Machines, just like HRMs and even calorie expenditure calculators are all estimations of your calories burned, the least dependable being your gym equipment, especially if they are not calibrated on a consistent basis. As for your heart rate monitor, if it has a chest strap I would use that if not you could be picking up a lot of interference. I always err on taking the lowest figure, tis better to underestimate calories burned.
As for your second question, when there isn't an exact activity listed you can use general low impact aerobic fitness to track your calories.
I hope this helps!