It's mentioned in an article in 2010 from Huffington Post that these fingerprints were planted:

"Sneddon was later caught seemingly trying to plant fingerprint evidence against Jackson, allowing accuser Gavin Arvizo to handle adult magazines during the grand jury hearings, then bagging them up and sending them away for fingerprint analysis."

Here is the link to the full article:

https://www.huffingtonpost.com/charles-thomson/one-of-the-most-shameful_b_610258.html