It's very simple, like a wired speedo, you have a magnet, and a reed switch, the magnet trips the switch, sending a pulse. Inside the switch unit is a very small, extremely low power radio transmitter, so low in fact that from more then about 2m away, you would not be able to pick up the signal. Inside the speedo unit is a radio receiver tuned to the same frequency. Now each time the switch gives a pulse, the transmitter sends a signal, that the receiver gets, it gives a pulse to the speedo. Each pulse means you have covered the distance of one wheel rotation, from that it can track distance, given that speed is a relationship of distance over time, it can figure that out as well.

Like any other radio receiver the speedo is subject to radio frequency interference, RFI for short, there are thousands of electrical and electronic devices that generate RFI, including the speedo's internal computer. Now different speedos can be affected different ways, for example one may use amplitude modulation, where another uses frequency modulation, they can use different frequencies, even the information sent, one might use a simple tick, another might send a set of tones. One may use the minimum transmitter power possible -- to save on batteries, another might use the maximum power legal, to get a nice clear signal.

Now, RFI can do two things, it can overpower the signal from the transmitter - blocking it, meaning that some pulses are missed, or it can appear as extra pulses, either way your speedo has the wrong count, so the computed distance, and speed are now inaccurate.

If you need absolute accurate results, like with a training program, stick with the wired units, if your only looking for an approximation, then look at reviews for various units, and see if they had trouble. Realistically though, these units have been around for a while, I would expect as the technology matures, that manufacturers will be trying to deal with the issue of RFI. However the cheap $10 unit, based on a 5 year old design, is more likely to have an issue, then a $100 unit, based on last years design.

One of the biggest issues is pagers, they use powerful transmitters, and may drift frequency wise, they tend to be powerful enough that the pager will still work, even though the transmitter has drifted off a good deal. Because pagers use a short burst of signal, and it may be hours between pages, it's hard for regulators to catch them at it.