No it can't be any of the above, the inlet pressure is the same (40psi), inlet tds everything the same.
spoke to my supplier today and he told me its quite common for this to happen, as the water temp decreases the membrane contracts, tightening the pores of the membrane making it harder for minerals to get through, essentially like going from a 20micron sediment filter to a 5 micron filter, it won't allow as much 'dirt' into the pure water.