@AstroChuck: Great question. As far as I know, this is a pretty complicated problem, and I think the human sex ratio imbalance has yet to be sorted out.
I have a book at home that might be able to answer these questions better as I’m having trouble gaining access to relevant articles, but I believe that male embryos are hypothesized to actually be weaker than female embryos. My understanding is that the likelihood of getting an X versus a Y is not, in fact, exactly equal, so you start off with a bunch of extra boys at conception and they slowly die off throughout life. (Yikes!)
Here’s a bit from the encyclopedia brittanica (although I want to point out that they do NOT cite their sources, unlike wikipedia!):
Studies indicate that male embryos suffer a relatively greater degree of prenatal mortality, so that the sex ratio at conception might be expected to favour males even more than the 106 : 100 ratio observed at birth would suggest. Firm explanations for the apparent excess of male conceptions have not been established; it is possible that Y-containing sperm survive better within the female reproductive tract, or that they may be a little more successful in reaching the egg in order to fertilize it. In any case, the sex differences are small, the statistical expectation for a boy (or girl) at any single birth still being close to one out of two.
http://www.britannica.com/EBchecked/topic/228983/human-genetics/50732/Fertilization-sex-determination-and-differentiation