Yup, you have to have practice with the calipers in order to be able to use them accurately. In fact, when getting your bodyfat tested with calipers, assuming the individual knows what they are doing, you should use teh same person each time or you might get a different reading even in the same day.
The electrical ones are inaccurate up to 8% and that's a huge margin of error. I've had myself as far off as 6% within a couple of hours. My lifting partner, Jonesy, couldn't even get a bodyfat reading off of it and he's carrying around 7%.