.....
The timetable is the same but the duty lengths are not, if we worked the same duty every weekday we could be on anything from 7 hours a day to 9hrs45 a day, 4 of those would either be a very short or very long week, so to balance it out we work 3 or 4 different shifts to get the hours near enough.
One of our worst weeks is-
Sun 15:50-23:50 (8.00) (you know the day we don't have to work, apparently

)
Mon 14:00-22:40 (8.40)
Tue 14:00-23:00 (9.00)
Wed 22:30-06:31 ((8.01)
Thu 19:45-04:45 (9.00)
Fri RD (yep we finish work 4hrs45 minutes into our 'rest' day!

Sat RD
Also note the short turn around on Thursday, finishing at 06:31 and back in at 19:45, brilliant for fatigue!