I've designed in a couple of AD7949 ADCs. In a typical 20-25C environment my boards give values that translate to temperatures in the range of 5C. The fact that they are all low makes me wonder if my calculation is wrong, but it looks fine. The parts are being used with internal 2.5V reference, b[5:3]=000.
Since there are no min/max limits for temperature measurement in the datasheet, I was wondering if this kind of offset is allowable. If so, then it would be appropriate for the datasheet to at least warn the user that calibration is required in order to get a usable temperature measurement.