Highly variable LR score

So sometimes I get -1, usually -2 or 3.... but there was one test where I got -7 and -9 on LR. I think it was PT35, very science heavy. Do you think this was a blip on a bad day or that I am in for some serious trouble unless I sit down with the LR Bible and study for October?

What causes such a wide margin on LR scores? How do you become consistent?