21st century is like legacy brand. Probably the oldest of the current brands including those direct marketing ones. It existed almost next to Scotts Emultion.

Well you got to see whats inside. Sometimes the milligram is less. Sometimes it uses different ingredients. Where the product is made from also dictates the price.

Next time you want to know, is by taking the picture of both the ingredients on the packaging and then compare it. Then you will know why it is cheaper.