This is where you are getting tripped up. You are correct that more CO2 would need to be released to pressurize a cold vessel than a warm vessel TO THE SAME PRESSURE. But that is not what is happening here. The cold soda bottle has a LOWER vapor pressure of CO2 than the warm one. There is no “equalization” of pressure here. As for your observed results, I don’t know what to say, perhaps some minor differences in the amount of liquid in the bottle, or one was older than the other and had therefore already lost some CO2 through diffusion through the PET plastic of the bottle can account for the discrepancy, but in any case it does not correlate with what should happen scientifically.