I am a fill main but sometimes people get autofilled on my team, why

Only thing i can think of is a "double autofill situation"

where the 5 best possible people to match have locked only 3 total roles.

Still not something i'd expect, what elo are you playing at for better reference? As higher elo/low level normal games have much smaller matchmaking pools.

Its because the matchmaking software already locked you in a position when choosing teams. When it comes time to pick the last position, if there isnt anyone maining that position, the position will get autofilled. You wont fill that position because you were already locked into a different position. Honestly, I just made that up, i have no idea how the matchmaking software works.

We have had some bizarre situations before. Like someone getting auto filled into a position that two people had as secondary.