Sion's ultimate is not scaling with his AD properly the wiki http://leagueoflegends.wikia.com/wiki/Sion (i don't know if it is accurate) It scales with .4 AD. I played a game the other night where it was .01 scaling I had around 100 AD and only had 1 bonus damage on the ult with the bonus AD it should be 40 bonus. (hope that made sense)
If you look closely, you'll notice that it scales with .4 BONUS AD, not TOTAL AD. This means that only the AD you get from items, runes, masteries, and in some cases champion abilities, are going to count for the bonus damage; your base AD won't count.
So, if you were only getting 1 bonus damage on Sion's ult, that means you must have only had like 2.5 bonus AD from items, runes and masteries (you could have had less and the tool tip just rounded up).
