240V/120V should not make double the difference but it's likely the 240V version might be able to put out a bit more current. Shouldn't be double and it's not a linear relationship.

It's all about power conversion - 240V at 1A is equal to 120V at 2A in terms of raw power. Then it's an issue of how efficient the power supply is. Inside the supplies rectify to DC and then using a switching power supply to output 5V. They still output 5V regardless of the input (within reason of course). As the DC rail will be lower in a 120V supply that can lead to less efficiency.

A phone/tablet charge is of the order of 1-2A but only at 5V i.e. 5-10W. A 120V outlet is perfectly capable of charging a mobile device at full power BUT they just might not be as efficient at the lower voltage on the wide range supplies.

I'm surprised you are claiming it's double the time. I might expect maybe 10-20% more depending on how good the supply is.