It tends to be more likely to happen on small projects, and is particularly likely to happen on projects whose goal is, "reimplement that".

However normally you're right, it doesn't work that way. Also anyone who has done some serious A/B testing of working applications quickly learns that small details are hugely important. Add the right words to a registration page and you can see a 20-30% boost in business. Put the wrong background on that page and you can lose 15% of potential customers. What are the right words? What is the wrong background? It isn't obvious - that is why you have to test. There is simply no way to figure out details like that up front.

Cheers,
Ben