I haven't seen a relay like that, with different ratings for the normally closed vs open contacts. No mention of it is made on the datasheet. The marking is probably wrong. It's rated to 15A at 120VAC, or 10A at 24VDC or 240VAC.
It's very common for parts to have a lower DC current rating, because a DC arc is more difficult to break, as the current doesn't pass through zero 100 to 120 times per second.