Why sometimes a stepper winding is not fully powered?
Constant power drive.
70% current in each of 2 windings gives the same power as 100% in a single winding.
(Ditto 50% current in one winding, 86.6% current in the other.) These correspond to sin/cos 45 degrees and sin/cos 30 degrees respectively, and sin^2+cos^2=1.
That's how you microstep between two full steps and keep the torque constant.
If you only use 100/0, 70/70 and 0/100 you are half stepping.
As stepper motors often run hot, they may be thermally rated for full power in only a single winding, or the same power distributed across windings.