[ahartman00]

Perception bias: is the tendency not to notice and more quickly forget stimuli that cause emotional discomfort and contradict our prior beliefs.

Confirmation bias: is the tendency to search for, interpret, favor, and recall information in a way that confirms one's preexisting beliefs or hypotheses.

"It appears to have something to do with both pre-suppositions and different orderings of personal values"

I believe the two cognitive biases above explain this. If negative numbers are "ignore", and positive are "use in conclusion": conclusion = -0.1 * fact1 + 0.9 * fact2 + -0.4 * fact3. Your past experiences determine the weights.

edit: I also think sibling comment by narag plays a part.