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Any activity where your wrist is moving will generate some steps and this, in turn, will affect your calorie burn.
Whether or not this fairly reflects the effort you put into Tai Chi is a different question. I'm not sure about Tai Chi but you can get better data for some activities by manually logging them using this procedure:
http://help.fitbit.com/articles/en_US/Help_article/1144
It might be worth trying and comparing the tracked data with the manually logged data to see if it's worthwhile.