C++ SDK Analytic Function inputReader.next() returns more actual rows than getNumRows()

I successfully created an analytic UDF using the C++ SDK but ran into a case that I can't understand.
For some partitions inputReader.next() returns more rows than I would expect from inputReader.getNumRows().

Which gives the definitive count? It seems from testing I need to write one output value for each row actually encountered from inputReader.next() not the expected number of rows from inputReader.getNumRows().

https://docs.vertica.com/11.1.x/sdkdocs/CppSDK/class_vertica_1_1_vertica_block.htm#a3093f20e18aeae81eb2afb4be5dbe5de

class WeightedMedian : public AnalyticFunction {
public:
    virtual void processPartition(ServerInterface &srvInterface,
                                 AnalyticPartitionReader &inputReader,
                                 AnalyticPartitionWriter &outputWriter) {
        try {
            // Get what inputReader thinks is the row count
            const vint reportedRows = inputReader.getNumRows();

            // Count actual rows ourselves as we read
            vint rowsRead = 0;

            // Read input and count rows
            do {
                rowsRead++;        
                 // do other stuff
            } while (inputReader.next());

            // numRows is 26421
            // rowsRead is 38345
        }   

           // Output value for each actual rowsRead (not getNumRows() which seems buggy)
            for (vint i = 0; i < rowsRead; i++) {
                outputWriter.setFloat(0, median);
                outputWriter.next();  
            }
}