Researchers have newly discovered a surprising and potentially significant reason why eating foods frequently cooked at high temperatures, such as red meat and deep-fried fare, elevates cancer risk. The alleged culprit: DNA within the food that's been damaged by the cooking process.
They are quoting the DNA concentration in g per kg of dry matter.
Plant cells have cell walls of cellulose, hemicellulose and pectin, which adds some dry weight that is absent in animals.
In addition, these seeds and tubers etc. we eat tend to be energy storage organs with a whole bunch of starch vs. not very much regular cell mass. It would be the same way if you ground up bone tissue and measured the DNA concentration.
Thanks for the detailed points and clarification :)