It's all a sly way of getting you for as little as possible, which inevitably wastes everyones time as you have the initial interview and ask about the salary to find it's a paltry sum.
It's also a way of widening the net to attract as many applications as possible.
We have position open currently which could be anything form £30-60k depending on the qualities and experience of the person who gets the job. We tend to look for people we like and fit the job to suit them rather than looking for someone with very specific requirements. If we said the job was £45k then that would put some people off for being too low and others too high so we'd miss out on some interesting candidates.