I imagine the difference is a combination of feeling and perceiving. There could always be verbal exclamations where the context and intensity are immediately evident but for the majority of cases it could be interpreted in many different ways. It may not even be truthful or sincere in the slightest. If I'm after romance I don't really want to have to go through all the possibilities (Fi-Si loop perhaps) and I want to know the conclusion. A hug expresses more and it helps to narrow down the possibilities. Of course if I'm not romantically interested either will do.
The poster who said it was a case of Fi vs Fe has a good point too, even though I have heard that INTJs are often uncomfortable with hugs.