What is it about speedtesters?

I was wondering if someone can shed some light on why online speedtest pages are so wildly different?

I am troubleshooting some connectivity issues I am having. During this process I have done speedtests using both nperf and speedtest.net.

Connecting to the same company servers - Devoli Limited, Auckland - I get the following results:

nperf - DL 30.18mb/s; UL 0.005 Mb/s; Latency 5.6ms

speedtest - DL 28.5 mb/s; UL 25.37Mb/s; Latency 4ms

The issues I am dealing with at the moment are:

Loss of Netflix

Inability of malwarebytes to connect to servers

Patchy connectivity

Primarily interested in knowing why the upload speed in these tests are so wildly different.

If anyone has any suggestions concerning the software issues, I would be interested in any insights into the issues. I have an asus RT-AC88U. Have updated firmware, rolled back the update, on forums etc. Not much success so far.