Mysterious Touchatag Data

So we know that the MIFARE Ultralight has a 64 byte EEPROM of which the final 48 bytes are for application data.
In the case of Touchatag tags, the data section is divided up as follows:

31 bytes - NDEF compliant URI that holds location of link (http://www.ttag.be/m/{UID}) (read only)
13 bytes - Mystery data; not in NDEF format (read only)
4 bytes - Initialized as all 0's (read/write)

Does anyone happen to know:
1) What that mystery 13 bytes represents? It seems to be completely different on each tag. Proprietary?
2) Was there a specific reason for the final 4 bytes being left writeable?  It's not enough space to insert another URI or anything functional according to NFC Forum standards.