It's not a dumb question - it is simply a matter of timing. The systems are set to trade during public hours, anything after that is finished with a T-trade. You can buy just before close of market, and the system will try to fill your order right up to the close. In the process, the MM may be short (not necessarily) right at the close. They can then cover after hours. They actually have an extra half hour to clean up their business for the day.
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