ARAM - random selection

Hello, I play ARAM mode almost exclusively. This mode is supposed to be completely random. So I cannot explain the following phenomenon to myself: Each time a group of 2, 3 or 4 people is in the match, I am associated with this group. When I say “at each”, it is well SYSTEMATIC (over more than a hundred matches). My question is: Why since the system is sane is totally random?

Aram refers to the hero selection and mode (All Random - All Mid[le lane]), not party size.

How else would duo, trio etc. parties get a game besides having the occassional solo queuers on their side?

If the team has two duo parties - there must be 1 solo player to fill the 5th slot.
If there is a party of 4, they must get at least 1 solo player on their side.

Also - how did you see party sizes? Enemy party sizes are no longer shown in the loading screen - only allied party sizes.
The only way you would see enemy party sizes in ARAM (since the final score screen isn’t shown) is whispering an enemy after a match, then checking whether they’re in a party from their current status - one by one.