One of the main reasons Mark Harmon could have remained credited was because of his behind-the-scenes role as executive producer.
It could also be due to contractual obligations, as the actor left mid-season perhaps bosses agreed he would remain credited until the end of the season.
Fans have also speculated it was a way for showrunners to honour Mark for the work he put in for 18 years.
As for season 10, fans will have to stay tuned to see if Gary will be the first star to be credited and if Mark will remain listed.
NCIS season 20 premieres on CBS on Monday, September 19 in the US. A UK release date is yet to be announced.
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