protocol design fail: MMS notification

I was just looking into some specifications of the openmobilealliance when I got the content for todays WTF moment.
An MMS notification is usually sent over SMS and encodes various fields including the location of where the MMS content is located so the mobile phone can download it via e.g. WAP.

Now looking at WAP-209-MMSEncapsulation-20020105-a chapter 6.2. (Multimedia Message Notification) there's an interesting header field included in these notifications, X-Mms -Message-Size

Mandatory.
Full size of message in octets. The value of this header
field could be based on approximate calculation,
therefore it SHOULD NOT be used as a reason to reject
the MM.

Clearly the people who developed this must have taken some bad drugs. Adding a length field value to a header and allow it to be based on an approximation rather than an exact value just doesn't explain itself to me.