Anyone can post on the ask, say you have 20 mil in YOUR account. You write up a limit order to sell at 0.0005. Later in the day none of them have sold, you say screw it and cancel your order. Now there is 20 mil less on the ask. However MM's have a little more power than us, as they actually make a lot of the trades happen on the OTC, so the broker calls them and asks to buy shares, if a bunch of orders start coming in, they can pull their ask in seconds before some of the orders are processed.
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