My mom's a really good cook and I'm a fairly good baker, and my sisters' explanation was that she likes to be spontaneous and doesn't like cooking strictly according to recipe while I like to be as close to a recipe as I can. That works very well for cooking but not for baking since baking requires you to be very exact and very much not spontaneous. Replacing ingredients works only for so long (white flour should never, never be replaced with whole weat flour for example...) and sometimes three minutes too long in the oven ruins the entire cake...

On the other hand, both my sisters are very good at both cooking and baking. Nyah. :S