The last print of the day and the official close are not always the same thing. The official price comes from the primary listing venue's closing-auction; a stray off-exchange print a second later does not change it.
This matters because trillions of dollars are marked to that number. A fund that must match an index is measured against the official close, so executing anywhere else creates tracking error.
Example: a fund holds 500,000 shares valued at the 40.00 official close, or $20,000,000. A 100-share trade at 40.35 prints on a trade-reporting-facility after the cross. The fund still marks at 40.00, because the late print is not the benchmark.
Related: closing-auction, market-on-close, vwap-algo, consolidated-tape