Usually there’s no technical reason. Older, discontinued cameras often have odd new pricing because retailers don’t always reprice leftover stock quickly or logically.

A store may have bought S90 inventory at an earlier wholesale cost and be reluctant to mark it down, especially if it can get manufacturer rebates, return unsold stock, or simply wait rather than sell at a loss. Different retailers also have different agreements with the manufacturer, so prices can vary in ways that don’t reflect the camera’s real value.

For discontinued models, the new price is often a poor guide to actual market value. The used market is usually a better indicator. In the examples given, used S95 prices were higher than used S90 prices, which is more in line with expectations for the newer model.

So: the S90 being listed higher than the S95 is most likely a retail inventory/pricing anomaly, not evidence that the S90 is technically better or more desirable.