Temperature readings differences?

Im using speedfan to monitor my CPU temperatures on my laptop. It reads 41C when laptop is idle. Speedfan also gives me a ACPI temperature reading which is 39C when laptop is idle (but fluncuates quite a bit...up to 44 sometimes for no apparent reason!) Both temperatures increase when i start playing games. The reading given by ACPI doesn't increase as linearly as the CPU temperature reading.

I also use Speedfan on my desktop. CPU temperature reading is 43 degees when idle. But the ACPI temperature reading doesnt fluncuate at all! It seems the first reading logged as the ACPI reading remains the value given by ACPI until i restart the PC again!

My first question is

What is this ACPI reading? Does it come from a sensor like the CPU reading? If not, how is it calculated? What is it used for? Should i take any notice of it?


My second question is

Why does the ACPI reading fluncuate with the laptop and not with the desktop?

Thanks in advance