"More ad units = more money" is wrong

"More ad units = more money" is wrong
Ok so everyone parrots "slap 6 units on every page." I did. RPM tanked.
Here's the thing AdSense auction doesn't tell you loud enough:
— each extra unit dilutes the per-impression value, advertisers bid on the impression not your slot count
— I went from 6 units to 4 (kept the high-value ones above fold), RPM jumped from $4.10 to $5.30 on the same traffic
The ad slots fight each other for the same advertiser budget. Density past a point eats itself.
Kill your two worst-performing units. Test it on one page first.