Your data is statistically wrongly calculated
For instance, 1998 August to December is +25.3%, not +34.2%. Many more instances, recheck your numbers
Going back to 1986, here's what August through December looks like in every midterm year:
• August: choppy, either a really bad month or a really good month. +4.2% median gain, but -1.0% average gain.
• September: bad month.
• October: +4.8% median, 80% win rate. This is the midterm rally.
• November: +4.5% median, 70% win rate. The rally continues.
• December: worst month typically with only a 40% win rate. Tax loss selling hits hard.
The total Aug to Dec return is +7.9% median with a 60% win rate, which is a good sign.
If this follows the historical playbook, expect chop through September, a strong October and November, then give some back in December.