Possible additional hypotheses: the reason there are more men born than women is because of selective abortions. If the selection pressure for having males is stronger in rural areas/ among the poor (where economics factors make it substantially better to have sons than daughters), and the poor have a higher mortality rate, then you would expect to see an evening-out. (It may be difficult to find good data on this, but I haven’t tried)
You may also find that men are more likely to emigrate, and if China has net emigration, then that would reduce the number of men overall. (There are other possibilities that rely on emigration, obviously. This seems possibly the most likely. Some research would probably be able to verify this)
In most countries, there are more women than men, because women live longer. (Some evidence: http://www.ssa.gov/oact/STATS/table4c6.html and http://en.wikipedia.org/wiki/File:UKDemographics-Age.svg )
Possible additional hypotheses: the reason there are more men born than women is because of selective abortions. If the selection pressure for having males is stronger in rural areas/ among the poor (where economics factors make it substantially better to have sons than daughters), and the poor have a higher mortality rate, then you would expect to see an evening-out. (It may be difficult to find good data on this, but I haven’t tried)
You may also find that men are more likely to emigrate, and if China has net emigration, then that would reduce the number of men overall. (There are other possibilities that rely on emigration, obviously. This seems possibly the most likely. Some research would probably be able to verify this)