Explain why the first ionisation energy of phosphorous is different to that of sulfur

Sulfur has an atomic number of 16, phosphorous is 15. The first ionisation energy of a periodic row usually increases left to right. This is because number of protons increases thus increasing nuclear charge. However, all electrons in sulfur are paired, whereas phosphorous has a lone electron in the 3p orbital. Paired electrons repel and so less energy is needed to remove an electron. The force of electron repulsion is greater than the nuclear charge increase and so phosphorous has a higher first ionisation energy than sulfur.