My math professors told me that math majors tend to fall in love with the second upper-division math course that they take, because in the first one, you're still struggling with the details of moving from mostly computational to proof-based problems. Usually this first course is either real analysis or abstract algebra, and since all analysis and algebra classes tend to build on the fundamentals of these courses, this may show up in the subsequent classes that you take within these fields.
The math tidbit is interesting though. What other examples have you seen of good mathematicians being lopsidedly talented in one area? What kinds of areas were these? I can somehow see real analysis involving a different ability than that of discrete math.
Yes. When I was getting a math degree, people who were really good in one area, like analysis, often struggled in another area, like abstract algebra.
Likewise, when I lived in Boston I worked with PhDs in microbiology and biochemistry who had a hard time with computer programming.