Usually if the degree is in the same subject area, only the highest degree for that subject is stated for post nominal letters - this same idea carries over to CVs in some peoples minds. It depends on the degree(s), but for the given example the only way to study an LLM is to have an LLB or equivalent Law qualification. Therefore it's already assumed they have that background if they have an LLM.
Now, if they did say, Physics as an undergrad, but Maths as a master's (or vice versa), then it's more likely they would list both if applying to say, industry, or an academic position in mathematical/theoretical physics for example. In that case, they're different courses, but they are both relevant (potentially) for different reasons, so interviewers etc may want to see their background in both areas. You can go into either from the other (in theory) but neither is a necessary prerequisite for the other. Similar to the above case though, if they just listed a master's in Maths, it would be reasonable to assume they studied the same as an undergrad.
It's really a stylistic choice more than anything, and in some cases where putting both doesn't add anything, it can help the overall clarity of the CV to not include the "redundant" information - since ideally it should fit on a single page. But it's up to the writer, and of course they can always explain in interviews etc if there is confusion. Also these days most positions require applicants to enter all their educational background individually into a webform or similar, as well as include a CV. So it'd be accessible to them regardless in those cases as well.