Another question about fuel

[Phobus]

Greetings,

I posted a earlier about running out of fuel after I had dived.

Now can some one answer this question. From my present patrol.

The situation, I am running at 6kts. Wind 15 m/s 189 deg
1538 hrs request Nav for duration. Answer 14,521 kms
1948 hrs request nav for duration. Answer 13,521 kms. difference 896 kms
2359 hrs request Nav for duration. Answer 13,166 kms. difference 459 kms

I am traveling from 'A' to 'B' in a straight line at speed of 6 kts. distance covered in first 7 hrs 21 mins circa 42 nm.

So why has my duration dropped by 1,355 kms?

I could expect some descrepancy due to the wind factor, but this amount?

At this rate I could well be out of fuel again before getting home.