If you check Google search engine ITA (https://matrix.itasoftware.com/), you will see that they offer a field at the very bottom that reads 'Sales City'. See also the respective Help page (click 'Help' and search for 'Sales City':
Due to airline business rules, the currency, fares, taxes, fees, and
flights available for sale can vary by city. Before searching, make
sure that the sales city is accurate so that you get the right
information for your location. If left blank, the sales city is set to
the departure city of your flight search. Click the Learn more link
for additional detail before choosing an alternative sales city.
That alone proves that there are intended differences in offers, depending on where you search and buy a flight from. The detailed differences of course depend on each airlines gusto and marketing decisions; sometimes also on their agreements - maybe in you case, Virgin has an agreement with Delta to not offer specific flights to customer starting on one side (and Delta vice versa); you'll never know for sure unless you work in these circles.
Fact is that it is intended by the airlines to offer flights sales location dependent for different prices or not at all.