Part of your assessment is no doubt correct. There are millions of "shy Trump voters" who either wouldn't admit to voting for Trump or wanted to mislead the pollsters. However it doesn't explain the many who Republicans left the fold, disenchanted with Trump, or who simply stayed home. Why didn't that have a more significant impact? The reason, I believe, is the tremendous voter registration effort undertaken by the Republicans that succeeded in filling their ranks with new voters, culled no doubt from membership in NRA or other single-issue causes. If the Democrats had been as aggressive in recruiting voters, I think the picture would look different.