Hi! I think the difference is that an extra standard attack isn't really considered a buff by the system but a skill that allows ninjas to attack a second time. This is why skills that allow for an extra standard attack would not stack because they only allow ninjas to do an extra standard attack from its original allowed number of standard attacks. Thus, having multiple skills doing the same, extra standard attack, will not increase the number of extra standard attacks.

This is why something like Han's mystery would not remove the extra standard attack, because it is not considered a buff that lasts an X amount of rounds.

Hope that helps!

~Jib