So to get the best answers, use Atari Basic Rev. C with FP ACCEL off.
And for Altirra Basic, turn on the Acceleration.
No, you should never use Atari Basic if you want the best answers :-P
The problem is that the programmers added a kludge to the power function that detects integer arguments, and in this case takes the integer part and adds 1 to the result. This relies on the math pack always returning less than the true result for the power function!.
In reality, this causes problems if you try to use the power function in any numeric algorithm, as it is now discontinuous and non monotonic, see:
If you are trying to plot the function or use a root-finding algorithm, or calculate the numerical derivative, the result will be really bad.
Altirra Basic does no rounding to the power operation, so the result it gives is the full precision of the math pack, see:
This "looks" worse, but it is actually much better if you plan to use the result on any further calculations.
Note that, with bigger numbers, the results of Atari Basic start to get a lot worse:
This should be really 67108864. Altirra Basic gives a better (nearer the expected) result: