I think I understand it now!
As part of the Report Descriptor, Report Id (4) defines a Maximum Contact Count usage that is defined as a Feature:
0x05, 0x0D, // Usage Page (Digitizer)
0x09, 0x55, // Usage (Contact count maximum)
0x25, 0x08, // Logical Maximum (8)
0x75, 0x08, // Report Size (8)
0x95, 0x01, // Report Count (1)
0xB1, 0x02, // Feature (Data,Var,Abs,No Wrap,Linear,Preferred State,No Null Position,Non-volatile)
As can be seen the Contact Count may be up to 8. So as Windows receives the Report Descriptor it sees that Report Id (4) has a variable field, and hence Windows asks the device 'how many simultaneous contacts do you support?' As the touchscreen supports 2 simultaneous touches, it replies with 0x04 (Report ID) and 0x02 (two touches).
I would be grateful if somebody could confirm my explanation.
[Some useful links that helped me find this information:https://msdn.microsoft.com/en-us/library/windows/hardware/ff553734(v=vs.85).aspx
https://msdn.microsoft.com/en-us/library/windows/hardware/jj151563(v=vs.85).aspx
http://janaxelson.com/forum/index.php?PHPSESSID=hmgmghq7mugbllnoejnpt5ugp0&topic=1284.0
http://www.usb.org/developers/hidpage/Hut1_12v2.pdf
http://www.usb.org/developers/hidpage/HUTRR34.zip
]