Stoploss / Trigger price

I’m getting confused with this term ‘Trigger price’.
Let me explain what happened to me.
I purchased Tata Steel shares at 1087₹ - 100 shares.

I created a GTT order to put both target and stop loss.
And in the stop loss, I see this ‘Trigger price’ and it says that Trigger price is the price at which my actual stop loss order will get activated and sent to the exchange.
I placed the Trigger price at 1078₹ and stop loss order at 1077.50₹.
So given the logic, once the price comes at 1078₹ the stop loss order is activated at 1077.50₹ right?

But I’m seeing the shares got sold at 1078₹ instead of 1077.50.
Why is this?!