I believe they are supposed to be 3Nm holding torque (i.e. when stationary).

Once that holding torque at 0 rpm is overcome and it starts to rotate there must be a sudden drop down in torque to ~ 2Nm, and then it is down hill from there? A bit like mechanical systems where there is a high stiction force to be overcome, and then the sliding friction force is much lower?

I'm no motor expert so take the following however you like (!) - but since they give the inertia load, and the voltage applied (which is much lower than the 70-80V that the motor and driver combo will happily run on by the way) maybe it is a measured torque under those particular voltage and load conditions, rather than the higher max theoretical stall torque at any given rpm. Not much of the usual knee point on the curve, for example ? So if they are selling that motor, driver and a cheap 36V supply then perhaps that is actually what you will get torque-wise. Happy to be corrected !

Anyway perhaps a different question that you want to know is will this motor work on my machine (with a given driver etc.) - if that is what you want to know you'll get lots of real world experience and advice, so don't get too hung up on the graph. It is a popular motor (if the inductance is fairly low) and works over a wide range of DIY machines.