In competitive fields ( like EM, Plastics, and Optho) you have to put in a ton of effort to show you are interested, and even then there is a high chance you may go unmatched from these specialties. So I'd suspect that the rate of unwilling applicants is very low in highly competitive specialties.
Whereas, in noncompetitive fields ( like path or FM), you can match with absolutely no interest or electives ( and even a CV that shows you had an interest in another specialty). So the no. of unwilling applicants in FM can be quite high. Also, based on this, to me at least, it seems that FM accepts applicants that other specialties has rejected ( so it is where less competitive applicants usually go).