This can happen in rare cases – it’s pretty rare though, and as in the case, the differences will be very small. A 0.09% difference is insignificant by every practical measure.
Best in Slot guarantees that it will get very close to the optimal solution, but it cannot guarantee to find the one best solution every time – the search space is far too large. It would take a very long time and a lot of CPU power to guarantee it (a prohibitive amount… we’re talking hours, not seconds). So we use all sorts of custom-tailored algorithms to speed things up.
When you lock something in, you’re giving the optimizer an extra piece of information, so it may find a slightly different solution as in this case – but also as in this case, they are so close in estimated value that it’s tough to say which would actually be better in-game.
Not sure if that makes sense… I can try to give more detail if you like. Gear optimization on the level that we do and at the speeds we target is quite complex.