Curiouser and Curiouser

I'm constantly looking at clothes online. One of my favorite online shops is ModCloth, widely known for having pretty unique clothes. I bought my leopard print jacket from them not long ago as well as a dress and I have a full wishlist that I have yet to send off to my mother. However, a little while ago I noticed a dress that looked familiar to me - I thought I had seen it somewhere else. So I kept checking around for it, but the dress disappeared. It was on sale this weekend and finally sold out, but I was interested to look at the price.
This is the dress from modcloth:
It originally was $52.99, but it went on sale for $26.99. That seems pretty reasonable until you see this:
This dress is from Forever21, and sells for $24.99. I'm not sure what happened here, I don't really know how manufacturing of clothing works. It's just something I found interesting - the same patterned fabric with the same design selling at two different places for different prices. What happened?