If you learn the difference between client side and server side you will know the obvious reason. In any moment the time is sended to server, the server calculates the difference time between when the page is accessed and when the captcha is sended. If is to easy like you say in all capcthas levels will have more solvers.
Then try to use them in a way that allows you to complete the level… I’m pretty sure you can’t complete captcha levels without programing a bit. Do you know js canvas, or ocr techniques/softwares?