The C range tires seen on the new Transits etc are a European "Commercial" rating not the North American letter rating. The tires on a Transit are more in line with the NA "E" rating. If you are buying tires it's becoming more important to look at the actual weight rating printed on the tire since we seem to be all over the place with ratings now on vans (Euro not Euro etc). Also, never buy tires that the size starts with the letter P...like P245/75R16. These are Basically passenger car tires (even in the larger sizes) and are no good for any kind of commercial work.
Depending on the vehicle the OP has, there is a possibility the particular size or size/brand combination is undergoing some consolidation and the manufacturer has chosen to only make it in 1 load rating to simplify things (and chose the heavier rating). However...a "G" rated tire is pretty darn heavy.
What size dancorn?