Stacked UPS

UPS218, a 767-300F ( reg. N330UP ) heading westwards at 32,000 feet. This was the third UPS 767 to pass overhead in two hours, each of them 2,000 feet higher than the previous. I wonder why, and why they are so low. Heavy with fuel and packages, perhaps?

Shot through thin cirrus, hence 'grey fuzz'

UPS218_B763_N330UP_IMG7223-2013-04-13.JPEG