What accounts for hucks surprise that tom will help him steal Jim?

Huck is surprised because Tom is higher in society. Huck cannot comprehend that a well educated and well brought up boy would "risk it all" by stealing Jim. Little does Huck realize that Tom is not helping Huck to break Jim out of the shed that he was held in, Tom is helping them for the adventure. That is why he adds ridiculous steps in the plot to break Jim out