We measure phosphorus (P) in the ICP test. The calculated value for phosphate (PO4) assumes that all phosphorus is in the form of phosphate. This is a reasonable approximation, as most of the phosphorus in the water column is likely to be in the form of phosphate as it is a very stable form.
There are likely small amounts of phosphorus present in other ionic forms that are more transitory. In practice, this probably isn't relevant for most purposes.
Phosphorus is used in many forms in living organisms, so what is important is the amount of free
phosphorus in the water column. In this case, using the calculated value for PO4 is accurate and relevant.
Also, keep in mind that most of the consumer-grade kits don't measure phosphorus or phosphate directly. They chemically convert a portion of the phosphate in the sample into another form that is easier to measure. As such, they often don't capture all the phosphorus or phosphate in your water column.
Hopefully this explanation is helpful.