That figure is some chosen figure I guess but not that important. When it is 69mm or slightly lower then it is ok.
I made a little drawing in solidworks, I don't know autocad, to explain what happens.
The first pic shows the bearing on the halfshaft and how to measure.
The second pic shows the halfshaft fitted in the casing and the rear hub over the halfshaft. As you can see the hub touches the bearing but the shaft is too far back and the thread isn't through the nut when fingertight.
The third pic shows a correct fitment after tightening. When tightening the hub is pressed against the bearing and the shaft is being pulled through the bearing till the conical part does seat inside the hub and the nut can be torqued correctly.
The fourth pic does show a too large distance of the bearing. Not correct.
The fifth pic shows what happens if the bearing is fitted wrong on the halfshaft. The hub is seated but does not reach the bearing. This way the load on the bearing is more because of the wider track. Also the brakedrum isn't centrated over the brakeshoes this way.
So the actual given size doesn't matter at all as long it is near. 69,92 wouldn't be a problem then.
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