‘Cause the Beeb says so! Scientists agree that if a kangaroo laid an egg from which an ostrich hatches, then it would be an ostrich egg and not a kangaroo egg. QED.
If you have some time to spend, read this list of 100 things we humans learnt/realized in the last year. The egg-fact comes from the list, as does the following, very interesting fact:
In Bhutan government policy is based on Gross National Happiness; thus most street advertising is banned, as are tobacco and plastic bags.