Why the demand for labour is lower than the demand for workers?

Why is it that the demand for one hour of labour (which is supposed to be a standard input) is lower than the demand for a whole worker, even if workers are always available to offer more hours of work one day (and employers are willing to hire more workers)?<br><br>Is this one hour of labour more valuable when it is part of a full employment contract as opposed to when it is a part of a part-time employment contract (or a part of a segmental employment contract)?<br><br>Given that the demand for labour is supposed to be derived from the demand for something produced by this labour, why is it that the demand for labour is not the same as the demand for workers?<br><br>Is it a discriminatory practice of employers that lowers the demand for labour, although it does not lower the demand for workers?<br><br>Is it a collective action problem that makes workers who are not available to work more hours one day (and employers who are willing to hire more workers) demand workers (rather than hours of labour)?<br><br>Is it a lobbying effort from employers that causes the demand for labour to be lower than the demand for workers?<br><br>Is it a consequence of the fact that the hourly wage is higher for full-time contracts than for part-time contracts?<br><br>[EDIT: I clarified the title of the question. It was previously "Why the demand for labour is lower than the demand for workers".]<br>[EDIT: I added the following question to the body of the post. It was previously in the comments. Thanks to user u/Dim-witted_dice for pointing this mistake out.]