Dwayne is correct here, Nathi.
The bone roll is mostly preference for this type of thing. Most often I too prefer to have the main axis for animation to be the X.
Having the X axis facing down for the arms in this position will mean that the main movement (forward and backward) on the upper arm will be the X axis, and the elbow will also move on the X. This is in deed much nicer to animate.
This is usually what I do but it was an oversight when putting this mammoth course together with all its pieces. It was too late into production when I realised I did this.
