It is common to find different jewelers promoting different natural gemstones as birthstone jewelry for a given month. They are both in the same color range and both have been associated with that month for a very long time. It is even acceptable for Jeweler C to suggest a Yellow Sapphire as an alternative, perhaps because the buyer is particularly hard on their jewelry and needs the hardness of sapphire.
Variations are perfectly acceptable, so long as the buyer knows what they are buying.
However, you may come across a display of birthstone jewelry meant for buyers who do not want to spend a lot to get the look of high quality birthstone jewelry. Often it is made from Synthetic spinels, which come in many colors. Most people don' t really care as long as the color is right, and they realize something must be up or the jewelry would be much more expensive.