What causes the issue?

This is caused by Product Listing Ads (PLAs), which have an average position of 0, but do receive impressions.

The average position calculation is weighted by impression count. The reason for this is to differentiate the performance of keywords with the same average position showing in the publisher over a certain period. The total average position is calculated by:

1. Multiplying the impressions by average position.
2. Dividing the figure by the total number of impressions across all objects.

This is expected behavior, and it doesn't require a fix. However, when viewing the average position for a group of objects, it is best to make sure you don't have instances where there are impressions, but no average position (i.e. PLAs, content, etc.).