We've often seen anything found through datamining be confused about whether the percent sign belongs in there or not.
My bet is that this is an 'up to +17 damage to moonfire' item, meaning that moonfire actually gets 15% of 17 added to the insta-damage, or 2.5, and 52% of 17 added to the DoT, or 8.8. Total expected damage, 11.3. Not that awesome but not terrible, especially if you're doing the moonfire spam by dropping the max-rank moonfire, then spamming a lower-rank moonfire.
My bet is that this is an 'up to +17 damage to moonfire' item, meaning that moonfire actually gets 15% of 17 added to the insta-damage, or 2.5, and 52% of 17 added to the DoT, or 8.8. Total expected damage, 11.3. Not that awesome but not terrible, especially if you're doing the moonfire spam by dropping the max-rank moonfire, then spamming a lower-rank moonfire.