Markup of new child item increasing if new child item is created for existing parent

Hi,

There is a parent item having 3 child items. The markup% of each existing child item and parent item is 74%. But, when we try to create a 4th child item, the markup% changes to 80% in the item-location screen, in-turn increasing the unit retail of the 4th child item at all locations.

Could anybody please guide me through this?

Thanks in advance!