Google Sheets natively controls how the functions recalculate according to different behaviours (modifying cells, opening the spreadsheet, …). This is why, sometimes, you enter your spreadsheet and see your functions recalculating and your data loading.
To avoid that IMPORTFROMWEB fetches the data source constantly, imported data are cached 24 hours by default (and up to 4 weeks). See on this page how to manage the cache.
Therefore, whenever IMPORTFROMWEB recalculates, the content is retrieved from the cache and it is not counted as new requests.
In case you want to retrieve the data and not to get updated at all (until you manually decide to update it), you can use the hard_paste option, which cleverly pastes the collected data as values directly into your spreadsheet.