This is about the approximation to pi not the approximation to float(atan(1))*4, it is exact (but irrelevant) for the later, for the former you loose two bits, so you have a 25% chance of correctly rounding towards pi.
This is about the approximation to pi not the approximation to float(atan(1))*4, it is exact (but irrelevant) for the later, for the former you loose two bits, so you have a 25% chance of correctly rounding towards pi.