I imagine there's some different legal wrangling and interactions with payroll tax thresholds and whatever inherent in a deal that is for X years with opt-outs vs a deal that is for Y years with option years that add up to X.
Like obviously not the same case since MLB doesn't have an NHL style cap setup, but I imagine that if you had a contract that was for $100m over 4 years with 3 opt-outs the AAV would be $25m even if the salary by year was $50m/20m/20m/10m, but if you had a deal that was like a $50m/1yr contract with 3 options progressively worth $20m/$20m/$10m the AAV on that deal for the first year at least would be $50m because options default to a state of not existing unless they are picked up whereas opt-outs default to a state of being part of the contract unless/until the necessary party voids them.