Well...the only way you'll know for sure is to ring them and ask them.
But at a guess, I'd say that what they mean is this.....
Lets say that they charge £150 per hour on the tacho. Then they are advertising their hourly rate at £120 (0.8*£150) on the assumption that you will only use 0.8 of the tacho in an hours flying.
That would seem low in my experience, and quite misleading, but it's only a guess.
dp