Max. switching current in relay
There is no 'power law' that applies to relay contacts.

When the contacts open carrying a current, an arc often forms in the gap between them, which damages the contacts.

AC current falls to zero 100 times per second (at 50Hz). This helps to extinguish the arc, which mitigates some of the effect of having a higher voltage.

DC current does not have this feature, less voltage is needed to maintain an arc. 

Current carrying, with closed contacts, is limited by heating in the contact area. This will be a function of the contact resistance, which depends on contact area, pressure, material etc. This joule heating is insensitive to whether the current is AC or DC. Therefore it's not surprising to see the same current specification for AC and DC.

You will notice there are two AC specifications, 250V and 125V. Obviously both cannot be 'right'. They are ratings. The point about a rating is that it's a test done under well defined conditions. What that means is when the relay is tested with some specified circuit, for x thousand operations, some specified parameter, perhaps on resistance, will not have changed by more than y%. If you want the detail of the test circuit and pass/fail conditions, then you need to track down the specification that this has been rated against. But basically, more voltage means more energy available to heat contacts, and more voltage available to keep the arc going longer.

Different relays have different contact materials, pressures, masses, speed and distance of separation, and these will all have differing impacts on carrying current and breaking current and voltage. Some relays may well have different rated currents depending on whether AC or DC is used, this one doesn't.