Temperature sensor on Marvell/Clearfog

On my Clearfog Base, I noticed that the temperature reported in motd was different than that reported by /sys/devices/virtual/thermal/thermal_zone0/temp - and reading the script at /etc/update-motd.d/30-sysinfo I found towards the end a comment about readings being done on the heatsink for Marvell systems (of which this is one), and the code subtracts 30 degrees Celsius from the reading.

I searched around and couldn't find any other mentions of needing to do that. I guess I'm curious how it was determined that this was necessary - and how accurate it actually should be. It seems like a questionable assumption that the sensor would report a temperature that was always a constant 30 degrees too hot - but if this is the only sensor we have I get that it's better than nothing.