The picking order in premade lobbies of 5 seems to be fixed

Hey there summoners,

over the last couple of months I have been noticing that the picking order for premade lobbies with five people on Summoner's Rift seems to be fixed. I've looked at every game and noticed the following:

• Whenever I play with four other friends and I get first pick around 80% of the time (based on roughly 200 games this season)
• It doesn't matter who I play with, therefore the premades themselves to not influence my position whatsoever
• When all five players used the "fill" option to determine their role, I'd be first pick everytime I have another role than support; if I happen to be the support I can land anywhere from 2 to 5.

Another addition to all players choosing "fill": There are stretches of games where I happen to be support for as many as 6!! games in a row ( the chances of this happening are 1 in 15625) smaller streaks like 3 and 4 happen way more than they should.

The fill thing is not a big deal because we end up switching roles all the time but the order in which we pick is rather important. I play a lot of mid lane and getting countered is not the most fun thing to go against.

I'd be happy over any response and would love to look into the mechanics that determine the order in which summoners pick their champions.

Side note: This only seems to occur consistently in 5-man lobbies, there seem to be some patterns for 4-man lobbies as well but I haven't really figured them out since I played like less then 20 games as four.

Thanks and good luck on the rift,

Shiny Rayquaza