Are three-phase contactors rated per pole or in total?
I think the calculation is not that easy because:


the power rating for (industrial) motors is not given for the input (electrical) power but for the output (mechanical) one. If you want the electrical power, you have to consider the efficiency ;
you have also to deal with the power factor which is not equal to 1.


I think the powers that are given in the datasheet you write about is the one you can find for standard motors. For example, here is a list of AC 3 phase motors from Emerson/Leroy-Somer:


For example, a 4kW motor at 415V (voltage difference between two phases) 50Hz has a rated current of 8.2A and a power factor of 0.81. We could calculate its efficiency:

$$ \eta = \frac{4000} {\sqrt{3} \times 415 \times 8.2 \times 0.81} = 0.84$$

Now, if we take the datasheet for contactors from Schneider (TeSys Green) (I think this is approximately the same products you gave the datasheet):


We can see that for a 4kW/415V motor, we have a 9A current. This is just above the 8.2A of the aforementioned motor.

Also, when you read motor datasheets, you will see that standard output powers are the same given in your document: 7.5kW 15kW 18.5kW...