Ha! I feel like such a simpleton now.
I just understood it as 'rotate 90 degrees clockwise'; clockwise, to the right, seems a reasonable direction to rotate. X and Y move from the top-left corner, tracking # pixels moved from that position: right and down, since there's nowhere else to go.
Sqiddster's talking about trig and I'm basically counting on my fingers. It just goes to show that Construct caters to all, even the "duh" folks. :)