How do you come at that number
Total bill was 144.77 THB / 43 Units = 3,36 THB per UNIT
(The 38.61 is the FT Charge (Fuel Surcharge)) which is originally 39.21 THB but you got a discount of 38.61 so actually FT charge is 0,6 THB (see the minus next to it)
It was indeed if you used less then 50 Units for 3 consecutive months (and you are on a 5/15A supply which is indicated by the # 1115 (under type)) that your bill would be 0 Thb,
We have the same issue with the house and land we bought of our neighbours, only have 1 light running 24/7 there and usage is 13 Units and still get a bill.
Some one told us that they stopped it (0 bill if less then 50 units used), but I will know for sure next month as then the 3 months consecutive is over.