It's a strange one Marshman and surely one that nobody really knows the answer to.
This little tale has reminded me of something my original 'mentor' once said to me. I had a first season terrier that really started well, he was going great and I ended up going on invites with lads to big old earths with a reputation about them, such was my belief in the young dog, and he always got the result. Anyway I remember being out one day digging and we were talking about an earth that goes to 12/13 foot deep, and this 'older lad' said I'd be able to dig it with my young dog, I said he must be joking, and he said young dogs that start so well are so full of confidence that they put in that extra effort and excel at work and it mellows as they get older?? It totally baffled me(and still does) but sounds similar to your cousin's dog?