You need to post links to the datasheets for both drivers so we can compare them.
Also, equally important, post a link to the datasheet for the stepper motor.
I imagine if the driver was set to 6 amps that it was putting 6 amps through the coils. Some drivers reduce the current when the motor is stationary - I suspect that is what the 70% refers to, but we need to read the datasheet.
...R