Thanks for supply that, as @retrotecchie has said your problem has been complicated by the fact that estimated readings have been used.
The first thing that I would do is to check the 4 bills ending 484, 613, 429 and 806. Check them one at a time starting with ...484 and compare the meter readings against what you have recorded. These bills will also state whether the readings were estimated or not.
Alternatively you could look back through your previous bills until you find one that has an actual meter reading, either a smart one or supplied by you. Then take a current reading and you'll know the m3 (I'm assuming your meter is metric) used over that period, multiply that by 11.5 and you'll have the approximate kWh used. Multiply this by the highest p/kWh for your tariff in that period and that will give you an approximate gas cost, then multiply the number of days between these two readings by an average of your Standing Charge. Add the gas costs to the SC costs and you'll have what you have used over that period, it's a crude calculation but it won't be too far out. Then add up what you've actually paid over the same period and compare the two.
It looks like your bills have been estimated for whatever reason, and on the low side. Once they have an actual reading they recalculate all previous estimated bills. Do as has been suggested and check your meter reading history.