When protected billing applies
A domestic connection can only be treated as protected when the account stayed at 200 units or less for the previous six billing months. That history matters. Looking at the current month alone is not enough, which is why a serious calculator should not auto-guess protected status just because the present bill shows low units.
Why the difference changes the bill fast
The protected estimate uses lower published residential rates for the first 200 units. The unprotected estimate is harsher because the month's full unit total is billed against the slab that matches the final consumption. That is why a jump from 200 to 201 units can materially change the payable amount.
What the main calculator does
The homepage calculator only applies the protected result when you actively confirm the six-month condition. That is stricter than many thin affiliate calculators and closer to the actual billing question that matters.