John, Mike & Pete,
There is a fair bit of discussion about this on the net – I really had no idea about it, however, there is a bit of video from David Attenborough, called ‘The Problems of Parenthood,’ which explains and shows this coot behaviour perfectly?

Pete - I think survival is usually expressed as young/ pair, not the proportion of young from a brood that survive. Which would equate back to the 1 surviving young per brood. This wasn't something I was aware of and it does sound bizarre.