Fajr time??

more or less answered at 9mins and 20 secs here:-

http://www.youtube.com/watch?v=NR54IMhKqLc&noredirect=1

Fajar Time according two time tables where the distance is a 5 minute walk.  The earlier one is at 18 degrees.

Unknown system

Sehri 1 / Sunrise          Sehri 2 / Sunrise    Difference in time

1.04     /  5.03               3.17    /   5.08            2 hours 13 min

1.12     /  5.04               3.18    /   5.09            2 hours 6 min

1.31    /   5.05               3.19    /   5.10            1 hour 48 min

1.42   /   5.07                3.21   /    5.12            1 hour 19 min 

1.50   /   5.08                3.24   /    5.13            1 hour 34 min

1.56   /   5.09                3.26   /    5.15            1 hour 30 min

And as you get towards the end of the month of Ramadhan the gap becomes lesser and lesser

3.22  /   5.42                 4.07  /    5.47            Less than an hour

3.25  /   5.43                 4.08  /    5.48            Less than an hour

3.28  /   5.45                 4.11  /    5.50            Less than an hour

3.31  /   5.46                 4.12  /    5.51            Less than 45 min

I'm very confused, however, I'd rather stay on the safer side and follow 18 degrees.

Any explanation for the decrease in time difference.

Jazakallahu Khair.