I'd assume because it's faster to move the index finger immediately up a row to the T when typing quickly than it is to move the ring finger up and over to the P. So chorded together, you end up hitting the T before the P to make GTP. Maybe it's a novel enough key chord that we can't rely on muscle memory to execute it well.

At least on my US qwerty keyboard, I notice this class of typo myself often enough.

We all have our quirks when it comes to common typos based on keystrokes. Not gonna speculate on spelling mistakes. Glass houses, stones, and all that.