Finger Spelling Alphabet

In the 3.1 Fingerspelling Part II I had noticed that the sign for the letter S shows the thumb as pointing up instead of crossing over the index and middle finger and also for the letter T the thumb is not being placed in between the index and middle finger, it is instead on the outside of a pointing Index finger and this is confusing as each time and place that I have seen these letters show they are always the same except on this particular lesson, what is the reason for this?