From how they measure the productivity score: http://blog.rescuetime.com/breaking-down-the-rescuetime-prod...
"But the bottom line is that the productivity score doesn’t tell you anything about what you actually produce. It’s simply a number that can give you an interesting baseline of where your attention is, but doesn’t tell 100% of the story. It’s a way to understand your patterns, and not a prescription for how you should be spending your time."
So, this is basically an indicator of how much time you spend on an activity and not how "productive" you are. Like they say, productivity is a charged word. I wish there was a way to tag productivity and efficiency together.