Relay - Datasheet Max DC Voltage
What's given as the "Contact Capacity" on the data sheet is the amount of current the contacts can break and achieve their life expectancy with the specified open-circuit voltage across them when the break happens. 

The voltage is higher for AC because even if the contacts open up at the AC peak when they're passing 1 ampere, at 60 Hz the arc/plasma generated by the contacts opening will automatically quench some time before the AC goes through zero volts, so less than 4.2 milliseconds from the break.

For switching DC, the open-circuit voltage limit is much lower than for AC because there is no zero crossing, therefore when the arc is generated it'll be sustained until the gap between the contacts increases enough to cause the resistance through the arc to increase to the point where the arc can no longer be sustained.

The criterion in either case is determined by the tiny area the contacts occupy when they're closed, and by the temperature rise they'll experience and how much metal will be melted when the contacts open/bounce.