YouTube API 429 error while not using full quota

Still a couple of weeks; we are getting more and more 429 HTTP Errors while making API requests to the YouTube API (mainly when trying to add or delete videos in a playlist). The error seems related to "quotas" indicated in the Google API console, but we checked this and are way below the limit according to the Google console report (we already have increased our quota value in the past, so this one should be more than enough).

Any idea why we are getting this error?

Error message:

{"error": {"code": 429,"message": "Resource has been exhausted (e.g. check quota).","errors": [      {"message": "Resource has been exhausted (e.g. check quota).","domain": "global","reason": "rateLimitExceeded"      }    ],"status": "RESOURCE_EXHAUSTED"  }}

And our quota for the YouTube API:

Update: We are doing a "pause" of 2-4 seconds between each request addition in a playlist and still getting the error.