Its an error from the calculator.

It’s because the calculator can only store so many digits of pi, and so when it comes across something that happens to be a multiple of *its* rational value for pi, it’ll jump the gun and assume that’s what you were trying to work out.

This is useful when you’re trying to say find an inverse trig value or the like, where you expect things to be in terms of pi, so it makes sense that the calculator makes these kind of errors in the attempt to include pi where it should.

I recommend the Classwizz 99-1EX if you are at the point where you want to calculate definite integrals by the way - it’s a little more sophisticated and has lots useful calculus and statistics functions.