Chip temperatures. I believe the board temperatures are accurate. I don't know for sure what the cause is, but I believe it has to do with the calibration of the external sense input of the TMP451 used on these boards. The external sense input is used to read the temperature of a sensor integrated into the ASIC, and needs correct calibration settings to get accurate results. The internal sensor on the TMP451 that is used to get the  "board" temperature is calibrated at the factory so it doesn't need any additional setup by the firmware.

What mikeywith mentioned is also true though. You may need different flow rates because of the different power density. A s9 running at 1400W is using only ~7.5W per chip. A T17 at 1900W is using ~21W/chip.