Average Joe sizes are cheaper than smaller and larger sizes

Went to MrP with my mom today, she was looking to buy some new pants. We found the pants she was looking for, but I noticed there was about a R30.00 difference between the two colours, now these pants are exactly the same cut, style and material, the only difference is the colour. Yes I do have photo evidence of this as well. the size 40 and 32 we could find on the rack were both marked at R279.99, but the size 38 was marked at R249.99 now the 40 and 32 are in a tan colour as marked on the tag, where the 38 that is cheaper is in as marked on the tag as tabacco, yet my mom found her size in both of these colours and both of their tags read R279.99. Once again I have photo evidence of this. How is it possible that the same pants in different colours could have a R30.00 difference to them? and no it's not only the bigger sizes that are more expensive, it's the smaller sizes too. The cheaper ones are the "average Joe" sizes. How are we allowing businesses to not have the same prices for the same clothing items?