The untested flash area may well be fully functional, but the emphasis here is clearly on "may". In the production of the chips, it is quite possible that individual blocks of the untested area have errors or quality problems that do not affect the rest, but are not found due to the lack of testing. The function of the untested area is therefore not guaranteed and can deviate in several parameters: temperature capability, number of write cycles, etc.
In addition, the next batch can always contain a further cost-optimised die in which the untested area has been removed and thus the silicon area reduced to the minimum.
Clear now?
Regards
/Peter
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