My wizard's an academic who is just starting her "field research," so I could easily justify increasing Dex simply by the fact that she's out doing stuff instead of hunched over tomes 24x7. That said, for an academic, increasing Int does make a lot more sense.
Well, depending on the particular academic institution (or the PC's particular teachers or mentors), he could have been trained in a regimen that included rigorous physical activity that played to his strengths- in this case, Dex-boosting exercises and drills, or participation in a school-sponsored solo or team sport.
Once he graduates, perhaps his love of that regimen means that he continues to improve both his Dex and Int over time, optimizing neither, but still becoming formidable in both areas.
Perhaps he'd even choose spells for which boosting his Dex makes sense, like those that required ranged touch attacks (rays, orbs, etc.). A boost of his Int still gives him better DCs and more spells, but the Dex boost makes him more accurate.
A RW analog of this would be the career of the late, great guitarist Shawn Lane. Known as one of the fastest (and innovative) guitarists of all time, one of his secrets was that he played piano just as much as guitar. He found that his practice time on piano helped him with his guitar playing.
[ame=http://www.youtube.com/watch?v=DhkbSBxPYcU&feature=related]YouTube - Shawn Lane talks about speed (Radisson Hotel, 17th Apr 1993)[/ame]