Robosmith had it. The power supply is not strong enough to heat both at the same time. It can maintain heat at the same time, but not increase heat at the same time. While printing you may notice this too. The bed will climb from getting low at a different time than the print heads. Also a heated bed will result in a lower maintained print head heat. With a non-heated bed, I stay within a couple degrees. With a heated bed, I'm lucky to stay within 5 degrees of the set temp.