A healthy outdoor rabbit can live for 8-10 years providing they are well looked after, good diet, taken to the vets when required, neutered vaccs etc
This is very true. The results are most likely skewed due to the "forgotten at the bottom of the garden" effect.
I once read an article that stated the average lifespan of an outdoor cat was only 18 months! Indoor was 12-15 years. You have to take into consideration the population they are are sampling. Was it feral cats, farm cats, pets that roam? It's still sad though.