Calculator Algorithmic Error (Bowmar MX140 [1974])
\$Y^X\$ is usually implemented as \$e^{X \ln Y}\$ so that X and Y can both be nonintegers. Logarithm and exponentiation are implemented using series expansions, and have some probability of not coming up with an exact answer.
Modern calculators carry out these calculations with enough &quot;guard digits&quot; so that the result can be rounded correctly for the display. Early calculators sometimes didn't have enough digits to round correctly in every case, and that's what you're seeing here.