What the page assumes
The converter uses a standard planning model of an 8-hour work day, a 40-hour work week, and a 2,080-hour work year. Those assumptions are widely used for scheduling, staffing, and budget planning because they translate one week of full-time work into five 8-hour days and fifty-two 40-hour weeks into one work year.
That does not mean every employer, country, or contract uses the same definitions. Compressed schedules, paid leave treatment, union agreements, and local law can change what counts as a work day, work month, or full-time equivalent in practice.