I also face the similar kind of problem:
When the table size is very small and contain a few rows only at that time this kind of problem happen. In addition to this when there is heavy insert/update/delete happen to the database i.e. few transactions per second at that time also you won't get Scan Density 100%.
One shouldn't bother much about the Scan Density on the small table.
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"Thare are only 10 types of people in the world:
Those who understand binary, and those who don't."