The part kits were previously using the cost set on the part itself, which caused discrepancies with the standard cost of the part from the warehouse. Now, the part kit includes a Standard Cost field, which is used as the part cost.
If a parts kit has a cost but the sum of the parts has a different cost, GL entries show a purchase price variance.
Set a default purchase price variance account on the Preferences page. (Setup > Company > Preferences)