Right, but it’s not just the scope – it’s a completely different combination of items and enchants. There are many trillions of combinations of items+enchants available, thus it is impossible to inspect every single one.
Optimization algorithms use a combination of heuristics to search a subset of solutions with very high probability of being optimal or near optimal. If you start giving different information to the optimizer (like locking in certain items or enchants) then it changes the subset of solutions that will be searched, and you might end up with a slightly different result.
It would be nice if the optimizer always found the exact same solution, and it was guaranteed to be the one best of the many trillions available… but it just isn’t feasible. It would be very slow and very expensive, and you would gain almost nothing from it. Two sets of gear that are estimated by a theoretical model to be within 0.5% of each other are for all practical purposes interchangeable – use whichever you prefer.