The following is based on current US usage (British usage might be considerably different): Sentence A would be generally understood to mean that Jack is arrogant and Tom is equally or less arrogant than Jack. There is also the possibility that Jack is not arrogant at all, and in this case Tom is not arrogant either, but who would even consider such a situation? Sentence B would generally be understood to mean that Tom's height is equal to or greater than Jack's height.
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