Answer
Planning variances arise from the original standard/budget being wrong (e.g., an outdated price), and are generally uncontrollable by operational managers; operational variances measure performance against a revised, realistic standard and are controllable. Splitting variances this way avoids blaming managers for flawed budgets and focuses attention on genuine performance.
Real-world example A market price jump creates a planning variance, while efficient buying against the revised price is an operational variance.