There should be two input bias currents, one flowing into or out of the inverting input and the other, the non-inverting input. Their difference is the input offset current. A typical datasheet only states one value for the input bias current. Is that bias current going into the inverting or the non-inverting input? Or is that an average of the two input bias currents? Or perhaps something else no specified on the datasheet?
If I assume \$ I_{B+} = 600 \text{nA} \$ and \$ I_{B-} = 400 \text{nA} \$ then I would get an average value of \$ 500 \text{nA} \$ and a difference value of \$ 200 \text{nA} \$. The calculation matches the values on the specification sheet where input bias current = 500nA and input offset current = 200nA. The leads me to assume that the single input bias current value provided on the datasheet is actually an average.
A similar question was asked here Input bias current in an opamp and its value in a datasheet but I can't find a relevant answer there or elsewhere on the web.