See 7158457 for more information, summary below:

The _avg_pretenured counter/value is sampled every time an allocation is made directly in the old generation. The value passed to the sample function is the size of the object in *words*.

When it's used in PSAdaptiveSizePolicy::update_averages() the _avg_pretenured average is added to another value, namely the number of *bytes* promoted. The units don't match.

The other problem is that the _avg_pretenured average is exactly that - an average. So the addition adds the average size of the pretenured objects with the number of promoted bytes. Chances are the code should add the sum of all pretenured objects since last promotion instead.