É porque WebDAV ("Criação e versão distribuídas na Web") é usado para criar a unidade mapeada para o URL do SharePoint .
No KB2386902: a unidade mapeada do WebDav relata a capacidade incorreta da unidade :
Cause:
...WebDAV protocol doesn’t support querying server’s disk capacity. With this constrain of WebDAV protocol, Windows client sets WebDAV mapping drive’s capacity the same as its System Drive.
Resolution:
This is by design. A WebDAV mapped drive's capacity is set to match the client machine's System drive capacity. This means that the capacity you get from a WebDAV mapped drive is the same as your local System drive. System drive is where Windows is installed.
Example:
On a Windows client:
System drive C:\
Free disk space: 39.9 GB
Used Disk Space: 11.2 GB
WebDAV mapped drive Y:\
Free disk space: 39.9 GB
Used Disk Space: 11.2 GB