The total hours in the workshop diary are calculated by the Opening times X Technician Efficiency.. e.g 8 hours in the opening times and 3 technicians at 100% efficient will put 24 hours in the diary, if a tech is 50% (and so on) with 8 hours set in opening times, it'll put him 4 hours in the diary.
Please see attached How To video, this includes a quick run through of viewing original total hours, amending opening times, then amending technician efficiency to see how they all reflect in the diary.