Had the same issue when mine was new. Over time the sensor will eventually become better. 1st summer mine read 8. Last year it got up to 11. This year I'm seeing 14 on a regular basis.
When my two year old UVN800 was new is was fairly accurate. Now it reads a little more than double what my Davis sensor reads. For example, at this time the Davis is showing 10.0 and the OS is showing 21!