The story, much like 2017, is most likely a story of differential turnout. Energized Democrats voted (as the Dem won the Election Day vote) while Republicans stayed home. However, this reinforces two stories.
First, non-Presidential elections are likely bluer electorates and the further down the attention scale, the bluer it gets.
Secondly, and building on the first, this is why the House is the leverage point. A triple the decisive amount of Republicans won in 2024 by less than 4 points. If there are consistent Dem overperformances in the special elections, it reminds them that they are extremely vulnerable in November 2026.