When evaluating a hypothesis or scenario, the evidence cannot easily be split into 'for' and 'against' arguments for separate consideration. Instead, all of the evidence needs to be considered as a whole. The final probability of the target hypothesis is determined by the likelihood of all of the evidence conditioned under both that hypothesis and its alternatives, and is not straightforwardly a summation of parts of the evidence.
As an example, take these three statements:
A: "Jones lives in Eton College"
B: "Jones is more than twenty years old"
C: "Jones is female"
Taken alone, (A) is certainly evidence against (B). Knowing that Jones is at Eton makes it highly likely that he's a boy or a young man. But if you know (C), that Jones is female, (A) becomes very strong evidence in favour of (B), since it's highly likely she's a teacher or the wife of one.
Items of evidence cannot, in other words, be treated in isolation from one another.