So, e,g, i'm trying to gun for say, KOS MOS, so it makes sense to put Zeke, and boost Justice. So I use 5 boosters at one go, (which means I have to farm boosters every single time), but I keep getting other elements, even when the boosters make the Light / Dark element way pointier than others. There was a video posted by some guy that made a bit of sense, e.g, if you're hunting for that one or two odd rare blades, (in my case, KOS MOS and Vale), three things can happen.

Stage 1: Justice is boosted, but it can still go other elements (like in my case)
Stage 2: If Justice pool is drawn (meaning light / dark, its a battle now between common blades and the rare blades, in my case since I already have Azami, its either Perceval, Vale, or KOS MOS, assuming if rare pool is picked). However, like in one video, the guy pulled around 20-40 wind elements before he found the last remaining rare wind element), all by boosting.
Stage 3: if the rare blade pool is selected, you're gonna have to pray if it lands.
TL;DR: too damned random.