[dba-Tech] Excel question -- another AutoFill question

Was playing around with AutoFill this morning and entered 1, 2, and 4,
expecting AutoFill to return 5, 6, and 8 -- but got 5, 7, and 8. 

I'm sure Excel is doing what it's supposed to do and the failing is my
understanding -- I don't think there's anything wrong, I'd just like to
understand why it gave me the results it did so I can get the results I
want. :) 

Susan H.