I guess the additional hill is what counts here, though it isn't very logical. I've noticed something like that, too, but with two tiles of forests - if the enemy stands on a forest tile and there is another forest tile between him and the archer, the archer cannot shoot, unless he is on elevated terrain. I suppose the rationale is that you need to have a clear line of sight between the archer and the enemy, i.e. no hills or forests (I think the same applies to jungles). Normally, it wouldn't be so irritating, as long as it was clearly explained somewhere (and it isn't).