In short, the problem is in the title. I realized, that search volume values, delivered by querying AdWords API are different (higher) than values shown by Keyword Planer GUI of Google Ads.
Language and location settings checked twice - they are not the cause. Aggregation should be not a cause too - Keyword Planer GUI shows values for each month of the last year, but no monthly value is as high as AdWords API is giving out.
Any thoughts about the difference?