Technically the bid of a buyer has to match the ask of a seller for a transaction to even occur! So you can't say "if it was on the bid it was a sell" it was just a transaction! They are all just transactions! There is no "buy" transaction or "sell" transaction they are all buyers buying shares from sellers! Every time. They didn't "drop it" to hit on the bid. When you place a limit order you tell the broker that price is the highest you are willing to pay, your "limit". They found shares below your limit. ie they got shares for cheaper than you were going to buy at! Thank your broker!
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