Why does lower atmospheric pressure produce higher EPR (thrust)?
You can also notice that for one atmospheric pressure, the EPR remains constant until it starts to get really hot.

That's a flat rated engine (or an engine that is not easily temperature limited).

The three limits for a jet engine are: pressure limit, temperature limit, and N1 RPM limit.

How can a lower atmospheric pressure produce higher thrust when temperature is constant?

Cold weather—high EPR is as you said is straight forward.

Normally a decrease in pressure would decrease the EPR. However, engine manufacturers are willing to push the pressure limit when the ambient pressure decreases (altitude rises).

This allows more thrust for a takeoff from a high airport, which otherwise would significantly limit the engine/airplane. This increased tolerance comes at the cost of more frequent checks for the engine, i.e., more money.


  We are frequently asked why this is the case. The engine manufacturers,
  recognizing that good takeoff performance may be critically important on the more challenging
  routes, allow some increase in the pressure-limited takeoff thrust setting at altitudes above sea
  level.
  
  Jet Transport Performance Methods - Chapter 15