FFmpeg: The option 'to' works differently depending on where we put it. Why is it so?

Could anybody explain, why

ffmpeg -i input.mov -ss 00:00:10 -to 00:00:15 output1.mov

produces a 5-second video, from second 10 to second 15, whereas

ffmpeg -ss 00:00:10 -i input.mov -to 00:00:15 output2.mov

produces a 15-second video, from second 10 to second 25?