The answer is fairly complicated, as such is the construction of the coil. from what I have been reading about transformers (a coil is a transformer) The core is not a single chunk of Iron, but many layers of thin steel. these layers are actually insulated from each other to make it more effective and are actually coated when new. but this coating is very thin, and with the humid conditions mower engines endure, rust comes through very quickly.
Contrary to what you would expect rust actually helps with the insulation between the layers. I have coils that are half as thick again with rust and still work just fine. People get all excited and clean the rust off everything, thinking that they are making it better when it actually does not change a thing..
