Yeah, weather will have an impact on boost. Dunno what the difference in heat was during the day/night in your area... but mine is usually decent... the day it'll be 26-30 degrees celcius and 'll get the exact boost I set it to. At night if it hits 15 celcius then I'll hit about .5 higher but not 1.5-2PSI difference like you're getting!
But I don't have any exhaust mods... so I'm less prone to spikes...