Because 20k miles seems utterly ridiculous and sometimes the unit (km or mi) gets left out of the post.
20k miles or 2 years is a fairly standard OCI in Europe.
The exact interval can vary depending on usage as its condition based servicing.
When I bought my car it had a 17k mile interval, after 50k miles in my hands the service interval is now 20k miles.
I am surprised that somebody with a BMW specific forum name is so lacking in knowledge of how BMW service intervals work.