I believe bleed air has to be cooled before entering the cabin, which under some conditions could condense some of the water content out resulting in the low humidity experienced (building air conditioning plants will cool air below the target and then warm it back up to ensure it isn't too humid). I'm guessing the 787 cabin compressors don't heat the air so much, so perhaps lose less moisture this way? But I can't imagine them carrying a tank of water to actively humidify the air?