Pretty much all math in brewing is based on apparent attenuation, though it is relatively easy to convert between apparent and actual.
The differences you are getting are just a result of when and where you are rounding your values. For example, 81% attenuation of 1.064 is not 1.013, but 1.01216. Your formulas are fine (though sometimes people use (OG-FG)/7.53), but they're just estimates, and even estimates are only as good as your accuracy/precision of measurement.
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