I read somewhere (no idea where now) that the male female ratio is dependent on the environment at or before tupping time. If there is lots of food available and the environment is comfortable then it's in the interest of the sheep to have mainly (and this is the bit I can't remember!!!) one particular sex of lambs. If there is little food and the environment is poor then its better to have more of the other.
Something to do with increasing the gene pool .. I think it was more rams if the environment is good as the result is a flock with a wider gene pool which helps the flock increase in size in order to make use of the plentiful resources. And more girls if it's bad as that means that more lambs will be born and so, based on the assumption that the mortality will be high, there is more chance of more lambs surviving.
This could all be total nonsense though
. And if it's true it's probably the environmental factors of more than one year that get taken into account.
I had 62% ewe lambs this year. Previous year was 50-50, year before that was 70% boys. All with different rams though. I'll be using the same ram next year so I'll be interested to see if I get a similar %.