I think this is because the V600 is a flatbed, which I have read is not the best for small negatives. My little plustek (which is very cheap) scanner is designed specifically for 35mm and does a great job, but then it can't be used for medium format. So there are always tradeoffs unfortunately.
You could argue it's all moot anyway, as scanning at super high res isn't gaining any more resolved detail. But I wonder, maybe the extra pixels actually do help when you add sharpening to the image, keep it looking more natural? That could be all in my head.