I wouldn't have questioned him about it either. The state gets to put up rebuttal witnesses. My best guess is that they wanted to rebut the defense witnesses testimony with someone accepted by the court as an expert.
If they had that witness, they would already be on the witness list. And if they did, why not be proactive and bring it up during the prosecutions case to minimize the impact of it. Ie, here is what the files show and here is exactly what it means despite what the defense will claim. I doubt they have a rebuttal witness for this.