What is overnight express?

So this is the second time I am expecting a parcel through Globeflight. The merchant I shop through pays for ONX delivery but it takes two days at least to be delivered. When I phoned the local depot about this, I was informed that all packages are routed through JHB before getting shipped out to wherever. That is apparently why it takes an extra day. She also told me that your rates regardless of which service is dependent on weight. Meaning I can order overnight or same day or economy and pay the same since it only depends on the weight? Now my question, as I posed to this person, was how can overnight shipping take two days? Also I just checked your website where it clearly states packages collected between 8am and 3pm will be delivered next business day before 11am. i got my shipping notification yesterday at 14h30, my package is still in JHB? How does this work? I am just trying to understand how overnight means two days?