I'd assume that limes are less acidic, which allows any sweetness to shine through.
This study I found suggests that to be a fairly good assumption, though the differences seem small.
idiggory, King of Bards wrote:
Though there's also a lot of mentions of sweet limes and sour limes. And I've never once seen anything but "limes" in groceries, so who knows what the hell they're talking about.
http://www.yumsugar.com/What-Sweet-Lime-2888122 wrote:
Although the assortment of produce seems to be more varied when the weather is warmer, I encountered an unusual fruit on a recent trip to the farmers market: the sweet lime. Also referred to as a Palestine sweet lime, the citrus fruit is native to India, where it's used medicinally to allay upset stomachs, nausea, and throat infections. It's still somewhat obscure in America, but the hardy lime, which blooms in Winter and Spring, has long been commercially grown in Vietnam, Egypt, along the Mediterranean coastline, and in states such as California and Florida. To learn how sweet limes get their yellow color, read more.
Generally, limes are picked while they're still green, but they actually turn yellowish in color when completely mature. Likewise, green sweet limes, while unoffensive, have yet to reach their full sweetness. Rather than having a high amount of sugar, the fruit contains less acid than ordinary limes, causing it to taste sweeter. Although the fruit's pale yellow flesh is praised for its mildness and palatability, some have described the fruit's taste as insipid due to its complete lack of tartness.