Lead scoring - why are some score activities missing from the lead activity log?

I'm in the process of refining our lead scoring program. To do this, I'm looking at leads and activity logs to understand how leads get certain scores. 

What I'm finding is that the activity log doesn't always allow me to work backwards to see how a demo score was assigned. I'll see some activities, but not all to add up to what the lead score is presently.

For example, I'll have a lead with lead score 30. When I go into the lead's activity log, I'll see activity for "changed demo score from 15 to 30," but I won't see the score change from 0 to 15.

There are several other examples of this I'm finding.