Yes, or on the other hand they may just be fed up of waiting for the lamb that's "not right" to keep up / catch up ?
It's either intuition or impatience. I'm not sure which.
I suspect you've also met the occasional ewe who has had a protracted and exhausting/painful lambing, who doesn't want to suckle ANY lamb, and won't let milk down despite a fairly full udder. She may be up and eating fairly well, but still "not right"; oxytocin doesn't help much, and then, nearing desperation, you try giving her a pain killer/anti depressant, et voila, she starts feeling better and behaving normally.
In my experience a first time lamber is much more likely than an older ewes to reject one lamb of twins, possibly because her milk production potential is more limited, and without exception the lamb she wants to keep is the better one of the two at that time. Just because sheep don't rate highly on the human IQ scale doesn't mean they can't be very good indeed at picking up signs of the less viable lamb.. The case where I think they sometimes get it wrong on a longer term basis is when the larger of an ill-matched pair of lambs is very slow coming out, and then quite dopey for a day or two until the brain damage resulting from anoxia during birth repairs itself. I've certainly never been able to detect any long term demerits with such lambs.