Hi,
At the bottom of the Pages report, the total unique visitors shown are those who had a value for the report in view. Visitors who did not set a page name are not included in the total on the report.
The Unique Visitors report on site metrics is representative of all visitors across the site, whether s.pageName was set or any other variable.
You may confirm this by making a segment to exclude visitors who had Page not null, which leaves only those visitors who have never set a page name. Compare the Unique Visitors report to the same with the segment applied. Those visitors in the segment account for the difference, the approx. 20k you mentioned.
Lastly, the interpretation is probably that some visitors are only triggering s.tl() link calls. The segment mentioned would help you troubleshoot. In typical production implementations across most sites, the Pages report unique visitors are going to be nearly equivalent to the whole site metrics, so something atypical is occurring here in this dev environment.
Andy