How does isopropyl alcohol disinfect less in higher concentration?
The main reason why alcohols (isopropanol and ethanol mostly) can be used as disinfectants is that they denature (bacterial) proteins. This is also the reason why they work on such a broad spectrum of microorganisms (bacteria, fungi, viruses), but not on spores, as these are better protected.

The higher the concentration of the alcohol is, the faster this denaturation is happening. And this can cause problems: When you denature the proteins of the bacterial cell wall, they will form some kind of protective layer around some of the cells which allows their survival. Diluting the alcohols makes the disinfection happening slower and thus gives the alcohols more time to get into the cell. 

For more information about concentrations and also for a ton of references, have a look into the CDC's disinfection guidelines in the subchapter on alcohols.
More details can be found in this review: 


Antiseptics and Disinfectants: Activity, Action, and Resistance