Very simple.

its maths.

Cause selangor the first few m3 of water is FREE.

Anything above that we pay.

But now they smart, "We will HELP the people by not charging them monthly, we will charge by combining 2 months bill"/

So suddenly your usage spike cause last time use 10 free 8m3 you pay 2m3 monthly. Now use 10x2 = 20m3, free 8m3, you pay 12m3.... congratulations