Percent-change metric alert reports unrealistic deviation

### Environment

SaaS (https://sentry.io/)

### Steps to Reproduce

Percent-change metric alerts are reporting wildly inaccurate deviation values. With a maximum deviation threshold set to 20%, alerts are firing and reporting values like 113% higher compared to the same time one day ago.

However, inspecting the p90(span.duration) chart over the full 24-hour window shows a stable, consistent signal between ~3000–4000ms with no gaps, zero values, or anomalous drops. The dashed comparison baseline tracks nearly identically to the current period. There is no point in the visible data where a ~113% deviation could be computed from the two windows being compared.

### Expected Result

The percent change reported in the alert notification matches the actual deviation visible in the span duration chart for the same time windows.

### Actual Result

The alert fires reporting a 113% increase in p90(span.duration) over the last 30 minutes compared to the same time one day ago.

### Product Area

Alerts

### Link

_No response_

### DSN

_No response_

### Version

_No response_

Reproduce the alert in Sentry SaaS using p90(span.duration), a 20% maximum deviation threshold, and the last 30 minutes compared with the same period one day earlier. Compare the notification's reported percentage with the full 24-hour chart; done when both windows produce matching deviation values.

Written by the indexing model from the issue text.