It might seem like chromosomal sex determination would result in a 50/50 sex ratio (at least, at conception--sex-linked differences in survivability may skew that ratio even before birth, as outlined in Will's answer).
Nevertheless, the human primary sex ratio (at conception) is just ever so slightly tilted towards females... and then switches by birth, to an excess of approximately 7 males for every 100 hundred females. Since males post-birth have higher mortality rates and lower life expectancies than females, it has long been thought that this difference is an evolutionary response to ensure that the sex ratio around reproductive maturity is close to 50/50. But whatever the reason, it's clear that just having chromosomal sex determination in humans isn't good enough, by itself, to ensure a 50/50 split. Yes, it helps to start there (where other determination systems may not), but there are clearly other mechanisms at play, and there's no reason those mechanisms couldn't force any other arbitrary ratio, if there's a good reason for it.
So, you don't need to worry about the underlying fundamental biological sex determination mechanism, or chromosomal structure, or any of that. What you do need to worry about is
a) Factors that would provide an evolutionary pressure towards skewed ratios, by whatever biological mechanism;
b) Over what sub-population you are measuring the ratio. Do you want skewed ratios over the entire population, regardless of age? Do you want consistently skewed ratios across all age groups? Do you want skewed ratios for some specific age groups? Each of these options may be selected for entirely different reasons!
Since the question mentions more males being born vs. females, or vice-versa, I'll presume you don't actually care that much about total population figures, or ratios at other specific ages. In that case, the obvious pressures would be extreme mortality for one gender over the other; if, for example, men die at a much higher rate that women, for innate (e.g., genetic health) or social (e.g., war) reasons, but the species is generally (serially) monogamous, you'd expect to see a lot more boys born. But if, say, women have extremely high rates of mortality in childbirth, you'd expect the opposite--most babies would need to be girls, to make up for the ones who die early.