It is probably a rolled roofing or SBS roofing that has been sealed over using a white-coloured sealant that you can see around the flashing of the chimney.
The house already has a low pitch roof but there is a transition of the roof pitch to an even lower angle as you come down from the peak to extend the roof the additional ~10 feet required for the room on the backend of the porch. The street-facing gable runs down to the eave just to the right of the porch; The long ridge runs, unbroken from the front to back, so to cover the room behind the red doors you have to change the pitch at the point the porch gable butts the main roof slope.
So quite literally, the mismatched roofing is the slope face that has a lower pitch or angle.
Once a pitch on a roof is low enough (<4"rise per 12"run), water run-off is not quick; water will loiter around longer, pool, and potentially back-up under traditional shingles. Additionally, low pitches using traditional asphalt/composite shingles can make it easier for ground winds to catch and damage the shingles.
To avoid this, a rolled roofing or a SBS roofing system was installed and the odd white colour is simple the sealant they used on the seams. Or they used the white sealant to repair an aging membrane system. A lot of the sealants can be applied with a roller or mop and it never look 'good'.