Why does Microsft Project leave empty days after leveling?

"I don't know why it happens but I can reproduce the problem.

When I've been forced to use MS Project for this level of micro-management, I usually model the support tasks with resource availability. So if a resource is only available on 3 days of a working week, I make the resource a 60% resource, with a comment telling that 40% of the work time is spent on support tasks."