It all comes down to troubleshooting by process of elimination for problems like this, so here's some suggestions.
This may have to do with which CPU governor or power profile the OS has selected to use:
You can change the power profile used if you install:
sudo apt install cpupower-gui
You'll see a small icon on the taskbar, click that icon and select which profile to use. I've set up one profile that pegs the CPU cores to 400 MHz (minimum frequency) with TurboBoost disabled, one profile that allows the CPU core speed to range from 400 MHz up to the TurboBoost frequency, and one profile that pegs the CPU cores to maximum frequency.
It may be that the sensor used to report to the OS and the sensor to control the fan are different, and the one used to report to the OS is miscalibrated (or the OS isn't reading it correctly), reporting a lower-than-actual temperature. You'd have to check your sensors to be sure.
Ocka also provided you an excellent suggestion... use a vacuum cleaner to ensure your heatsink fins are free of dust, use a pipe-cleaner to get any stuck dust out, also make sure your fan blades are dust-free (especially the leading edges). Don't over-rev your CPU fan with the vacuum cleaner, it can ruin the bearings.
Some BIOS / UEFI setups have the ability to change the 'ramp-rate' for the fans. If yours does, check that you've not set the fan to ramp up quickly at a lower temperature.
One thing I've done for my laptop is to get two Mountain Dew bottle caps and place them under the rear of the laptop to lift it off the desk a bit. The extra clearance gives better airflow to the fan, so the fan runs slower. At idle, the fan shuts down completely. On my old laptop, the surface was so slick that it'd slide across my desktop and slip off the bottle caps, so I epoxied them to the bottom of the laptop... the new laptop has a little silicone grippy strip across its width at front and back so it doesn't slip, and the back grippy strip traps the bottle caps so they can't move.
No... that's not just an excuse to drink Mountain Dew. If your SO protests, tell them it's a legitimate need... and you'll need spares just in case. 
