Reverse Mathcology.
If you want something to do a certain thing...you have to tell it do the opposite.
In this case, desired result = average going down. To obtain that, you need to do the opposite...eg: shoot above your average...which you did. Quite simple, really.
Kidding aside, it's due to the formula used to calculate your average. It only uses your best "X-number" of rounds in the calculation. If that "X-number" included a 39 or higher, your 38 would replace that score, and your average would thus come down, even though the 38 is higher than your current average.