Why one stepper motor is twice faster?
Comparing the specs of the two motors side-by-side, there are some significant differences:

Model 17HS13-0404S1 datasheet
Model 1704HS168A-OB datasheet  

Both are standard NEMA-17 size mounting plate.
Both are bipolar connection.
Both are rated for 12VDC operation.

Specification       | Model 17HS13-0404S1   | Model 1704HS168A-OB   |
Amps/Phase          | 0.40 Amps             | 1.68 Amps (DC)        |
Resistance/Phase    | 30.00 Ohm +- 10%      | 1.65 Ohm +- 10%       |
Inductance/Phase    | 37.00 mH +- 20% @1kHz | 2.8 mH +- 20%         |
Holding Torque      | 0.26 N-m = 2.65 kg-cm | &gt;= 5.5 kg-cm          |
Step Angle          | 1.80 degrees          | 1.8 degrees +- 5%     |
Rotor Inertia       | 38.00 g-cm2           | 68 g-cm2              |


The 17HS13-0404S1 has significantly higher winding inductance and resistance, and thus takes lower current -- this directly implies lower torque available.

One of the key differences between a stepper motor and a brushless DC motor (BLDC) is that the stepper motor has a Holding Torque, because it is optimized for holding a position, rather than constantly spinning.

The speed of a stepper motor is really a function of the controller that drives it. A stepper can be run "open-loop" giving very slow pulses. Assuming 1.8 degrees per step, the controller can determine the shaft position just by remembering how many pulses have been given. However this only works when the pulses are slow and the rotor turns fast enough to keep up.

2500 steps per minute * 1.8 degrees per step / 360 degrees = 12.5 RPM

When driving a BLDC or a stepper motor for smooth continuous rotation (at low speed, or without Hall effect position sensors), the controller cannot just cycle through the sequence without knowing where the rotor is. The magnetic field should always be "leading" the rotor's magnetic field, otherwise if the rotor is too slow then suddently the magnetic field is "behind" the rotor, and torque is negative. Often the symptom is that the motor "sings" or vibrates instead of rotating. The solution is to slowly accelerate the rotor. However the rotor's inertia must be taken into account. So it's not surprising that a different motor (with different winding inductance and different rotor inertia and holding torque) would need to be tuned with a different acceleration profile.

I had not encountered grbl before (https://github.com/gnea/grbl), looks neat, I may want to dig up my old stepper motor test bench and try it out sometime...