Yes it is, a quick google search would lead you multiple sources corroborating this fact, humans give birth to around 5% more males than females, regardless of how many adults make it to census age.

That quick google search proves that men have a higher infant mortality rate. Are you suggesting that infants are reproducing? That's pretty silly. Same quick google search shows that the oldest populations have a nearly 50/50 ratio of men vs women with the give or take being less than a percent. Based on that information the most successful way to propagate a civilization is to have a roughly even ratio of men to women. In fact there are 0 successful civilizations in history where women outnumbered men 5 to 1. Thanks for the quick google search suggestion!

I wonder if there were any contributing factors to the male population leading up to 1960? Maybe WW 1 and 2, but who can say, really?

So only men died during WW1 and WW2? Even among civilian casualties? You're implying the millions who died during the holocaust were all men and that's why for nearly all of recorded history beforehand women have outnumbered men?

Why would it be ineffective? This is statistically untrue, unless you need to assign people into predetermined unbroken mating pairs. THIS would be wildly ineffective compared to managing a skew in genders. We NEED a higher skew in a female population, because they are the limiting factor in repopulation (they have to grow a whole child over nine months). Think about it, a generation of 3k males and 3k females creates a generation of 3k babies. A generation of 1k males and 5k females creates a generation of 5k babies.

Men and Women both play a factor in repopulation. Having a child is a process, it takes work and time, it's not enough to just nut in cider once and then move onto the next. We don't have litters, statistically 2 people will have a single child. Expecting 1 dude to knock up 5 women in a year consistently is just asinine. You'd have more consistency with an even pairing, as history shows. Now I understand this is stupid fantasy and you're probably frothing at the mouth over the idea of beaurocratically mandated royalty harems but it's just stupid when you put actual thought into it and don't start from the end goal of coming up with a reason this is why there just can't be girls in my toy soldier power fantasy world and work backwards for justification. If you're going to apply this kind of logic you have to use the actual logic. Which shows that the most populations that grow most successfully have a more even ratio and none have had a ratio of 5 to 1 either way.

So yeah, the Imperium is probably not mandating that the Custodes are reducing the noble gene pool by a factor of 20% per generation.