I have been blowing the whistle for 20 years, The Slide tackel is one of the hardest things to call right, you need to see if definder got all ball, was cleats high, was the definder late, and do all of this in less than a second. We get it right most of the time, and sometimes we get wrong, And the other thing to consider. Setting in the stands you have a different view, plus sometimes we are partially block from view by other players. All we can do is call what we see.