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davidcyclebackPSA said a 6.5 means the card is an extra good example of a six. A .5 essentially means a '+.' They could have just as easily used a .4, a .3 or a + to mean the same thing. So the .5 isn't intended as an exact mathematical measurement, but a notation that card is a premium example for that grade. PSA's numeration system isn't something you'd want to use to perform actual mathematical calculation with.
If PSA could literally determine where .5 was, that would mean they would know where .4 and .6 are. You can’t count to five without counting to four first (and you can't count to four without counting one, two and three). You can't know where is exact center without knowing exactly what is on each side. If PSA could really determine where is dead center, that would mean they could provide 90+ different grades.