You've got it. I don't have a picture of my actual wiring, but here's the idea: circuit ab has 2 resistors, R1 and R2, in parallel while circuit cd has 2 resistors, R3 and R4, in series. Resistors in series add the way you think they should: Rtotal for circuit cd is = R3 + R4, so when you put resistors in series, you increase the total resistance in the circuit. Resistors in parallel are squirrely, they add together, but in an odd way: Rtotal for circuit ab can be found using this equation 1/Rtotal = 1/R1 + 1/R2. In effect, adding a resistor in parallel reduces the overall resistance of a circuit.

So, a 15k ohm thermistor acts as a 15k ohm resistor when the temperature is 25 C (that's how they are rated--the resistance they provide at 25 C). By adding a 25k ohm resistor in parallel, the circuit now shows [1/15k +1/25k]^-1 = 9375 ohms which my a/c unit thinks is about 30 F hotter than it actually is.