Why are all North American-spec E28 M5s officially 1988 models if the last example was made in 1987?
This was most likely a marketing decision. With the M5 originally intended to be a limited edition of 500 cars, and with the entire E28 range due to be replaced by the E34 for 1989, perhaps BMW of North America wanted to keep the E28 M5 exclusive by offering it only for the final 1988 model year. Also, it is worth noting that production of the other 1988 5 Series models began in early 1987, so the M5 preceeded these by only a few months.
