So how does the matchmaking adjustment in team builder actually function?

Playing Irelia to mixed success in blind pick; sometimes I do well, sometimes I get crushed, sometimes I lose lane but come back later, sometimes I roflstomp my lane but then the team never wards and I get to watch a slow throw, the usual.

Fire up team builder and it says "you're new to this champ, role, and lane, we'll scale it down for you" and then I get a Nasus who has 350 stacks at 14 minutes and a Fiora who deals double damage to Irelias (so far I stomp the Fiora's I get in blind pick).

Because if what it actually does is it says it'll find someone exceptionally higher than your usual skill level so you can "learn" from them, it'd be nice if it actually said that. I get the same sort of disparity when I play adc's. I can sometimes do well in blind, never in team builder.

What is going on here?