The influence of cloud drake
For most champions in the game, their ultimate is their strongest ability. Cloud drake offers cooldown reduction on ultimates.
Chance of at least 1 cloud drake: 75% Chance of only one cloud drake (in a cloud drake game): 67% Chance of multiple cloud drakes (in a cloud drake game): 33%
Chance of only 1 cloud drake overall: 50% Chance of multiple cloud drakes overall: 25%
So in half of all games, 1 team is going to have 10% shorter ultimate cooldowns than the other.
Now let's take a look a what champions are doing well in each position and see if they benefit from CDR on their ultimate:
Top: Garen, Nasus, Mordekaiser, Malphite, Yorick, Dr. Mundo, Urgot, Quinn, Kled, Singed
Jungle: Tryndamere, Warwick, Nunu, Ekko, Jarvan, Ivern, Elise, Olaf, Vi, Nocturne
Mid: A Sol, Heimerdinger, Annie, Malzahar, Kassadin, Rumble, Ahri, Cassiopeia, Kayle, Ekko
Bot: Heimerdinger, Senna, Jinx, Ashe, Jihn, Kog Maw, Miss Fortune, Veigar, Caitlyn, Twitch
Support: Kled, Heimerdinger, Syndra, Katarina, Ahri, Mordekaiser, Malphite, Fizz, Rumble, Akali
Almost all of the above champions get huge value from CDR on their ultimate. Then there is the ultimate hunter rune, which may be taken.