I think that this might be more due to the number of individual attributes related the the special skill; that is those that are boosted. There more attributes there is to be boosted, the more skills points will be are required.
For C.K (40), the expected related attributes are mainly crossing (minor: creativity).
For F.K (40), the expected related attributes are mainly shooting.
For P.M (50), the expected related attributes are mainly passing and creativity (minor: crossing).
For D.W (50), the expected related attributes are mainly positioning and bravery (minor: strength).
For H.S (50), the expected related attributes are mainly heading and positioning (minor: strength and bravery).
For 1-on-1 scorer (50), the expected related attributes are mainly finishing and dribbling.
So you can see that those that have 2-3 main attributes require 50 s.p as compared to those that have 1-2 which require 40 s.p. There might be also minor attributes involved which explains why the difference in the required number of skill points is only 10.