lscpu and /proc/cpuinfo shows constant cpu frequency

Question

However, such script

 watch -n 1 'for i in $(seq 0 7); do cat /sys/devices/system/cpu/cpu$i/cpufreq/scaling_cur_freq; done;'

shows that turboboost is working and everything fine.

The strangest thing is that BIOS shows constant cpu frequency too !!! (3400Mhz)

Setup:

Ubuntu 16.04.4 LTS

4.13.0-36-generic

i7-6700

UPDATE

After update to the latest version of a BIOS (2018) nothing has changed.

UPDATE

The same thing is happens on my laptop with identical setup except CPU - 7820HQ.