We had a similar problem...in fact, it was exactly the same. We were getting 257 also and wondering how that was possible. Well, turns out the sensors were working fine, it was just printing the wrong numbers, if that is possible. Try printf-ing one at a time. I'm sure there is an explanation, I just have no idea what it is.
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MIT Mechanical Engineering
>> College Mentor,
Team 97: Cambridge Rindge and Latin School with The Edgerton Center, MIT Mechanical Engineering, Bluefin Robotics, and Draper Laboratory
>> Alumnus, Team 527: Plainedge HS