Perhaps the "sugar" in the soda is actually the much-dreaded high fructose corn syrup? I'd suspect as much, since this study was done recently, and most sodas don't use "sugar", per se, any more.
So both juice and soda have a good deal of HFCS.
You simply need to correlate the results to other figures to see if HFCS is the major variable sticking out.
source: http://en.wikipedia.org/wiki/High-fructose_corn_syrup