The reason why is because the spinner has 12 slices and on that spinner there is 12 yellow slices meaning all the slices on the spinner are yellow.
This means it is a 100% chance to land on yellow but 100% aas a fraction is 100/100.
You can simplify 100/100 a hundred times to 99/99 to 98/98 you can keep doing it but it's lowest form is the lowest number besides zero and that is 1 so it's lowest form is 1 out of 1 or 1/1 or 100%.