Having tried this a couple of times, if the Transfer Pumps are never switched on, the Supply Tank contents start reducing at about 250kgs Main Tank contents. So, if that was the case, they wouldn't have been up as long as they were.
Thanks! That were also my suggestions. Above 250 kg the fuel could flow via the overflowholes from main to supplytank. But below that level, you need to have at least one XFER pump!
So in this accident at least one of the XFER must have been on in the first part of the flight.
skadi