The buy at market or limit order your provider offers uses a...

The buy at market or limit order your provider offers uses a sweep strategy.

There are bids/offers in the midpoint of prices that you/we cannot see. The bids/offers always match off and execute, leaving it either bid or offered at the mid (either unfilled buys or sells). Your buy order sweeps through the midpoint to the offer. In this case, there are/were unfilled offers in the midpoint (29.75c) so you get filled on whatever is there, up to the size of your order. If the midpoint was bid (unfilled resting buy orders) your sweep through the mid would fill nothing until it bought/lifted the offer (30c).

Hope that makes sense. Let me know if it doesn’t and I can attempt to re-explain. It’s basically ensures best fill price on your buy limit orders.

The only time people may not like the order type is if they hope to lift the whole offer creating a run on price.