Thanks everyone, that is all really interesting!
I had no idea that these were a desirable type of egg. For poaching I definitely prefer them to be more viscous so that they hold together. I'm not sure what the cause is because we have some older hens and some younger, and I think these odd eggs...
I wonder if anyone can help. We have 7 hens and often poach the eggs. Some eggs are perfect but every so often we get some where the yolk is almost completely separate from the white, so when you poach you get the white with the yolk sitting in a perfect ball on top and almost completely...