isnt the screenshot already explained?
-RM106 is upfront payment
+RM143.65 is the 1st bill charges (the current charges total missing the +RM0.02, confusing to read

)
total balance to pay is RM37.65 lor, since upfront payment not enough to cover the 1st bill charges ma. simple maths.
the right question to ask is, why RM143.63 charges? go read the bill details in the selfcare.
here's from from what i calculated:
prorated webe mobile = RM139.30 (10 Jun - 30 Jun, 21 days, both dates inclusive)
prorated #webenow = RM-84.00 (10 Jun - 30 Jun, 21 days, both dates inclusive)
advance webe mobile = RM199.00 (1 Jul - 31 Jul)
advance #webenow = RM-120.00 (1 Jul - 31 Jul)
additional usages = RM1.20 (to be confirmed by you, read the bill charges)
currrent charges = RM135.50
6% GST for RM135.50 = RM8.13
total = RM143.63 =>
RM143.65 (+RM0.02 rounding adjustment to nearest 5sen)upfront payment = RM106.00total balance to pay =
RM143.65 -
RM106.00 =
RM37.65note: from estimated calculation above, did you make any calls to special number or IDD or premium SMS, total worth RM1.20? fyi, local fixed+mobile charges are FOC.