These arise because, according to modal realism, reality as a whole – that is, the totality of the posited worlds – is necessarily the way it is. Lewis is very upfront about this. Witness:
There is but one totality of worlds; it is not a world; it could not have been different. (Lewis 1986: 80.)
So, for example:
'If there had been two fewer men in reality as a whole than there actually are, there would have been fewer women.'
There is no reason to think this is true. And yet Lewis's thesis about counterpossibles, together with modal realism, implies that it is vacuously true.
'If there had been fewer men in reality as a whole than there actually are, there would have been just as many men in reality as a whole as there actually are.'
This seems positively false.
Thanks to Quentin Ruyant for pointing out that the last counterfactual, given modal realism and the thesis that there are infinitely many worlds with men in them, actually seems to come out true in a funny way: if there had been two fewer men, there still would have been infinitely many. So this was a bad example. Consider instead:
'If there had been no Model-T Fords in reality as a whole, there still would have been some Model-T Fords in reality as a whole'.
Lewis, David K. (1973). Counterfactuals. Blackwell Publishers.
Lewis, David K. (1986). On the Plurality of Worlds. Blackwell Publishers.