"...the type of empathy the study was referring to was not emotional empathy but cognitive empathy. Cognitive empathy is the ability to understand what someone else is thinking. This is entirely different from emotional empathy, which is understanding what someone is feeling..."

From tumblr. We can't tell what we're feeling very well a lot of the time. But we are EXTREMELY good at knowing what others are feeling. This is due to detecting their micro-expressions, due to being hyper-vigilant. A lot of people with BPD had abusive childhoods and they had to be on high alert at all times.