Why was I charged for cancelled Apple Music subscription?

I got a free trail option for Apple Music for 3 months and after those months there will be a Monthly pay of ₹99. Before the completion of my free trail I cancelled my subscription and now I just received a text that ₹99 are debited from my account… my question is why is it debited if I cancelled it before the start of the plan…

[Re-Titled by Moderator]