That just means your boiler is a lot less efficient at full pelt than it is at lower output, and this is counteracting the gains had from keeping the house temperature lower when unoccupied. Perhaps it's not condensing properly in those conditions due to insufficient temperature drop across the radiators ? As I don't have a condensing boiler I don't have this factor to worry about - efficiency won't vary much with return flow temperature or whether it's going flat out or not.
I did the same tests you did where I used a set back in the 16-18C range at night and during the working day for the full house and it was unequivocal in my case - gas usage was much lower just scheduling the zones to 5C when we were out of the house (and downstairs zones at night) and letting optimal start figure out when to start heating to get things back to where we wanted them again.
In mild weather like spring/autumn you'll find that when turned off for more than about 12 hours a house will reach an equilibrium temperature - for example it might reach 15C and stay there, so if your set points are below 15C in those conditions you'll use zero gas even if you're away for several days but if your set back was say 18C then you're constantly using gas during that period.