The explanation for this is in the Common Issues thread (Hearthstone Common Issues)
The Arena rewards are added when the run retires, which in this case was the second that Rise of Shadows released. They are not added when you click through the rewards screen, that is just the game revealing what was already added to your account.
Yesterday I had 1 HoS pack (I didn’t buy any bundle but did win one in Tavern Brawl). When I logged in the game shows 2 packs, even though I did nothing to get an extra pack. I then went to the Arena screen to see the rewards for my retired run and saw an HoS pack - which is the 2nd pack. I did not get a 3rd.
Especially people who did preorder and hence have a lot of packs to open do not always look at the exact number of packs - they just start opening. So it is possible you already opened the pack before even going to the Arena screen…