Talal,
As from Jan 1st, 2013, new rates are being applied so in this bill, they have charged you Rs 1491 for the days of December and Rs 4072 for January, 2013 consumption. So they have applied Slab 3 i-e Rs 500/MMBTU on December,2012 (app last 10 days) and also Slab 3 i-e Rs 530.69 on your January,2013 consumption. However as your consumption is less than 3 HM3 in this bill, slab 2 should have been used. As you can see that for Dec 2012 bill your consumption was 2.57HM3 and bill was Rs 2289. Therefor for 2,92HM3, your bill should be near this amount. Therefor in my view your bill is wrong and you should bring it to your revenue office and they will hopefully amend this. Let me know of the progress