The "market" is more complex than a single stream of matching buy and sell orders at specific price levels. For any given asset, there is an "order book" containing the quantities market participants are willing to buy/sell at various price levels. Lots of people coming in to buy at "the market price" eats away at the selling side of the order book, raising the market price level.
Perhaps buyers nibble at the ask. Perhaps a big order slams through. Or perhaps the posted liquidity is canceled without any trading having happened. Whatever the case, the number of shares sold is exactly equal to the number of shares bought.
