Well, Vanessa is much better at descrining this than I am, for sure, but I believe the common theory is that the logias are by default always in their elemental state, thus even when they are taken by surprise, they are still unharmed. I don't know how that explains the phenomenon of when they are able to be hit, other than when whatever it is negates the power (as in the water on Croc).
I still do not get how not being able to conduct electricity (as a rubber man) allowed Luffy to land punches on Enel. My personal pet theory is that there are interim stages between pure elemental state and fully solid that a logia user can develop over time and through training. However, I have never heard anyone say the elemental state covers the "real body" when it is in play, so saying that these folks "dodge" is the correct wording isn't really a proven fact any more than saying with absolute confidence that Teach can regenerate by absorbing the damage into himself through the yami yami no mi.
We simply do not yet know how this works, and I believe this is by design on Oda's part, since I think he is saving this info for later fights between Luffy and Logia users like Akainu and Teach. Much more exciting to have Luffy guessing what to do to win than to have all the info now, and have everything be a foregone conclusion when the fights come, don't ya think?