BART's explanation for this is that heavy overtime plus few employees results in lower total lifetime costs to cover the same area (of time or space) than no overtime and more employees. This is likely due to post-employment costs since most CA pensions are defined benefit not defined contribution.

I would be unsurprised if this weren't also true for other CA departments.

Though there is a likely positive feedback loop as increasing compensation with a fixed budget results in fewer hires, more work, and therefore more compensation.