Google - Maps

Type in an existing and non-existing address and compare the results.

Google maps does not claim to-the-metre accuracy. Geographical information about addresses, buildings etc, does not always match up to map data accurately. Google is making no claims about the existence or otherwise of addresses. How could they? They just process data supplied to them and present it.

I just typed in my own address and the map shows a pointer indicating the middle of the street just like you described.

In my street house numbers stop at 54. I typed in 406, My street, My town, UK, and got a pointer halfway up the road which bore a message saying "Placement on map is approximate". My (real) address did not have such a message.

THis suggests that Google is using British Post Office address coding information, and if a house number exceeds the highest number shown, it just gueses and adds the message I described.

The data Google maps works from includes the lat/long of the ends of a
street segment (block) and the address range for that block. It uses this
to extrapolate the location of an address within that range.