Well, suppose the 2 witnesses said it was a man, and suppose they even have a partial picture of him, that shows his clothes, and his short black hair, but not his face. And suppose the 2 witnesses never got a chance to see his face.
Then along comes somebody who's assuming it's Dede, and they're a little bit wacky, and they call in a tip saying "I saw Dede in the truck! I saw her red hair and everything! Yep! It was her! For sure!". Well, then LE knows the person is mistruthifying, and they don't need to waste their time on that tip.
Then along comes somebody else who calls the tipline and says "I saw a man in the truck! He had on a light blue jacket and a red ball cap with such and such logo on it, he had short black hair, and I saw his face. He had a mustache and bushy eyebrows and a big scar on his right cheek." LE knows this is somebody worth spending their time on.
LE has to know how credible tips are so they can prioritize them. One of the ways they do that is by witholding info. They know by what the person says - or doesn't say - how credible the tip is.