Its just maths.

If any number over 10,000 is pure white, if you are adding two pixels/one dual pixel readout it is the sum of the two, if each records 6,000 the accumulated reading is 12,000, a blown highlight, if you also record the half pixels you get a reading of 6,000, which is between 5,000 and 10,000, an extra stop of information in the highlights.

So any time the half pixel reads between 5,000 and 10,000 you get additional information. It's got nothing yto do with resolution, it is just the maths from counting the elections and the fact that any number higher then the highest possible is white, it doesn't matter if it is 1 higher or 1,000,000 higher, it is just white.