That could be...or possibly it really does feel sooooo good right at that moment that they have to comment on it. I know that every time my husband enters me I do the same thing...a sharp intake of breath followed by a little moan. That first thrust, in the heat of passion is a little bit like heaven for me. My response may seem predictable or even put on but I assure you it isn't.
Like an earlier poster said, if you are faking then you really can't complain if you're not "getting there". If you sound happy then what motivation is there for a man to change up his routine? If you are faking to stroke his ego...well, I suspect sooner or later he will catch on.