As others have posted, it's usually binding in the nut (or string trees on a Strat).
singemonkey wrote:
Is it because it's too thick for an unwound string? That's the only theory I've got.
The G is more problematic because basically it should be a wound string, but wound strings don't come thin enough for modern electric guitar sets (thinnest I've seen is an .017w). Modern string sets are balanced for a more consistent feel across strings, rather than tonality or stability. You can go slightly heavier on your G, which will improve tonality and tuning stability, but at the expense of flexibility and playability - it will feel a lot stiffer and be harder to bend.
Wizard wrote:
Why does the G string sound more "nasal" than any of the others?
In addition to the above, vintage-style Strat pickups with staggered polepieces usually have the vintage stagger - which was made for wound G strings, so an unwound G will jump out as louder and brasher.