Step 2: Determine which districts vote for you less often.
Step 3: Ensure that fewer ballots are delivered to those locations.
It is intentional, not accidental. They probably used low turnout from prior elections (due to voter suppression) as justification for not providing enough ballots for every registered voter.
The problem with the US system seems to be that it's partisan all the way down. It's too easy for the parties standing for election to influence how the election itself is run and counted. This is, I guess, an effect of the USA's highly decentralized approach to elections: if the Republicans run a county, they get to decide how elections work in that county. A more centralized system wouldn't leave the same scope for tweaking each local election to get the desired result in that locality.
Republicans also love to volunteer / run for local elections that oversee these logistics purely because they want to manipulate it in bad faith. It's SOP for them.
Hell, they have even been caught multiple times putting up fake ballot areas, and "helping" non-native English speakers fill out their ballots, and being in full control of delivering those ballots.