Kornacke on MSNBC did a segment on historical mid-term votes that showed how very unexpectedly well the Democrats did.

He pointed out statistics that showed this.  He used economic data (inflation, etc), Presidential approval ratings, and past mid-term elections and through these, the data/past elections indicated the Democrats should have lost approximately 50 House seats. 

For example, Biden's approval rating is around 44 percent, and the last times a President ever gained seats in a mid-term happened twice and that was when both presidents had approval ratings in the upper 60s.    He also pointed out when past presidents had similar approval ratings in the 40s, ALL lost big in mid-terms - Biden has not. 

Lastly, my thoughts are that Republicans LOST so many of these elections and Dems lucked into winning them and the Democrats need to realize that.