The magic hands of the Phone Guy debunked

I was promised (and am paying for) a certain bandwidth $X$. But sites like http://www.speedtest.net/ indicated a bandwidth $Y$ (much smaller than $X$). To gather more empirical data over time and to make sure, that I am not hallucinating, I installed a Zabbix server on a Raspberry Pi ((https://www.zabbix.org/wiki/Zabbix_on_the_Raspberry_Pi_%28OS_Raspbian%29)) and set up a monitoring for my Fritz!Box ((http://znil.net/index.php?title=FritzBox_mit_Zabbix_%C3%BCberwachen_HowTo_mit_Template)).

The data for the item "Fritz!Box DSL-Downstream" clearly indicated that $Y$ was around 6 Mbps. So I called my ISP and they ultimately sent the tech-guy to the rescue: He was at my home around 8:30 in the morning and measured the bandwidth with a small magical device. First at the basement where the cable enters the house and then at my office on the first floor. To my surprise, both measurements indicated a much bigger number $Y'$ (which was bigger than $Y$ and looks much more like $X$). He also left me under the impression, that everything was fine with my internet connection the whole time, which I might have believed. But:

https://blag.nullteilerfrei.de/wp-content/uploads/2015/03/netcologne-zabbix-bandwidth-1024x210.jpg 1024w, https://blag.nullteilerfrei.de/wp-content/uploads/2015/03/netcologne-zabbix-bandwidth-300x61.jpg 300w, https://blag.nullteilerfrei.de/wp-content/uploads/2015/03/netcologne-zabbix-bandwidth.jpg 1520w" sizes="(max-width: 1024px) 100vw, 1024px"> Note the time, when something changed! I rest my case. He doesn't have magic hands. He cheated!