IEC 60320 C13/C14 Voltage Rating
The isolation defines the voltage-hardness of any connector.

Basically, I wouldn't know how to build a C13 plug that was only safe for 125V; the pin distances definitely are large enough, and if you attach the conductors at the position of these pins, then these contacts are far enough from each other, too.

But I'm not in the business of making cheap connectors.

However, the most likely explanation for a "125 V" marking on that cable is that it comes from a supplier that targets a 120V market. Often, names matching what the customer expects outweigh technical specification in marketing.

(that's the reason why you can get "Samsung USB cables" on ebay, that are not built by Samsung at all. People have a phone and want to buy a cable. They type in the only text on that phone into ebay. Same goes for power cables. People live in a country with 120 V. They don't want to stand in an aisle and hold a cable that's rated for 240 V; that just confuses them.)