Lower = less demand.
Guess the single degree is just more popular than any double degree...
Honestly, I don't think this is the case. Almost everyone I know in first year is doing law as a double degree. I can probably count the people I know doing straight law on one hand...
Further, it doesn't explain why combined degrees like Arts/Law and Commerce/Law have essentially the same TER requirements as other less popular double degrees. I would guess at least 75% of first years at Monash do either Arts/Law or Commerce/Law, yet the high demand isn't reflected in the course entry requirements.
I think Monash just arbitrarily sets pure law higher than double degrees in law. Probably because they want to encourage students to take this path, or as a strategy to compete with Melbourne Uni's JD degree. Or it could be that they just view straight law as a harder course (because you have to do 3 law subjects a semester).