I believe the answer is the latter, but I'm looking for more detail, hence the question.
For low volume stocks and options I have bid lower than the current bid, and while usually my limit order isn't filled, it has been in the past. Is the market itself responsible for this (a retail investor somewhere sees my limit price and decides to jump on it), or is it the market makers, who if they buy, see my limit price as being within some tolerance that they don't publish?