Rust in one furnace, but not the other

does anyone have an idea on what could be causing this amount of rust in the below pictured furnace, while there is one of the same capacity(66mbh) , age(2011) and model right next to it that there is no rust. We did not install either of these furnaces, but there were inspected by our AHJ, have they own independent vents 4” b vent, and are installed in the same combustion zone. Right next to one another.

the first I serviced either of the furnaces was today. I did a combustion test on both with all readings within an acceptable range, and the proper manifold pressure

the only thing that I can think of is that the one with the rust is installed closer to the outside wall in a below ground basement that it may be subjected to a higher humidity level when the furnace is idle in the summer? The heater without any rust has the water heater connected to its B vent and maybe with just the pilot on in the WH that is keeping the vent above dew point at all times? The furnace with the rust has its own independent b vent. I checked , and both rain caps are in place on the roof. I am kind of leaning towards a humidity issue because if there was an issue with the vent, then why would the gas pipe in the furnace be rusting as well? Any suggestions would be much appreciated. Thanks to all