For example, most people will assert, when asked, that there are more English words ending with "-ing" than with "-g".

Is it possible that at least some of those people think (at least subconsciously) about words ending with the phoneme /g/ rather than the orthographic letter G? A famous experiment in which most people consistently skip “of” when counting the number of times the letter F occurs in a sentence (presumably because in “of” it stands for a /v/ sound) suggests to me that that might be not that unlikely.