BO5 1 win to get 100 LP. 3 wins out of 5 to promote. that's 4 wins in 6 games = 66%
BO3 1 win to get 100 LP. 2 wins out of 3 to promote. that's 3 wins in 4 games = 75%
And you say there's no luck involved.
It may occasionally need extra tries but if you don't have 3 and 4 game win streaks in your history you are an outlier and you don't even need to streak them consecutively bc you have the loss of two buffer. They take a higher win percentage but they buy you some level of safety at the next level. It is amazingly hard to drop from X5 to Y1. So it makes sense that you have to pay for that safety with a higher win rate over the short term.
