I apologize if I sound dense, but doesn't the term 'may always act in a surprise round' basically mean that he is always aware regardless? From what I can tell, that seems to be the case, but it doesn't answer the question: what does 'is never surprised' actually mean in gaming terms.
For example, lets say a bunch of enemies ambush the party and a surprise round begins. The party already made their perception checks in order to determine who was 'aware' of the ambush, and any players that made the DC will get to act in the 'surprise round'.
However, a 13th level Kensai would get to act in the surprise round regardless of whether or not he made the perception check (he does this because of the ability gained at 13th level). Yet for this example, let us assume the Kensai is 19th level, and thus can never be surprise.
What happens then?
From what I can tell, the term 'never surprised' seems to suggest that while a surprise round does occur for all other combatants, the Kensai would treat the 'surprise round' as a 'normal round' instead (because he can never be surprised, and is thus not subject to the normal 'surprise round' restrictions). Based on the wording of the ability, this doesn't seem to be an inappropriate explanation, especially when you consider the inverse of a question:
How can a Kensai act in a 'surprise round' if he is 'never surprised'?