I wonder how accurate will the devices used at the competitions be. The better not use cell phone apps. I'm specifically talking about the frequency measuring device from the event or the event supervisor not the student's. There could be errors that will include a certain level of randomness into the measuring.
The rules for event supervisors should specify what device to get.
In a perfect world all ES would have access to high-quality measurement devices and use these for competition. However, this is not always practical or affordable. While I hesitate to speak for the rules committees, I think I am safe in saying that they make an effort to keep events accessible even for teams with limited financial resources. Therefore, they leave this open to ES to provide the best equipment possible; I would expect higher levels of competition (regionals, states, etc.) to make more of an effort to have good equipment than invitationals.