So, why would anyone think that? The notion that women have an extra set of ribs is probably based on a misinterpretation of some Bible verses in Genesis. The actual quote is:

But for Adam, no suitable helper was found. So the LORD God caused the man to fall into a deep sleep; and while he was sleeping, he took one of the man’s ribs and closed up the place with flesh. Then the LORD God made a woman from the rib he had taken out of the man, and brought her to the man.
(Genesis 2: 20b - 22, NIV © 1983 by Zondervan Corp.)

Notice what this doesn’t say. It doesn’t say anything about how many ribs Adam had before or after this “surgery,” and especially it says nothing, whatsoever, about how many ribs Eve had! Nowhere does it say that Eve had more ribs than Adam. Who made that assumption without checking? Wouldn't it be just as logical to guess (also without checking) that if she was created “second” that God might have made her with the same number of ribs as the new, reduced number that Adam now had? Wouldn’t it really be more logical to guess that God might have created her with the same number of ribs as Adam just to avoid confusion? Genesis doesn't say one way or the other, so the only way to know is to cut open cadavers and start counting. Who has done that — the theologians or the biologists? My guess is that, once again, if human misinterpretations are set aside and if it is remembered that theology looks at “why” while science looks at “how,” there is no “conflict” between what the Bible, itself, is saying and what biologists know to be true about our bodies.

Oh dear... at times, we let our logic thinking overtook our sense. I guess at times we just let our logics take advantage of our facts. :)