Tools infer volume from clickstream panels and from Google's own bucketed ranges, then average it over twelve months. That smoothing hides seasonality entirely: a query that peaks in December and disappears in July shows the same monthly figure as a steady one. For anything seasonal, look at the trend line, never the headline number.
Accuracy degrades outside English and on low-volume terms, where a reported 90 can be anything between 20 and 400. The correction is free: your own Search Console impressions measure how often a query ran while you were eligible to appear, which anchors any forecast far better than an external estimate.