Question about Sonnet Solo 10G TB3 to 10GB Base-T

Maybe someone can give me some information: I got the adapter today, and everything works fine so far. Now during the speedtest (Ookla Speedtest app on the Macbook): 2 flat white "Swisscom" long cables, one with yellow plugs, the other with red. I don't know what exactly these cables are, but I don't think they are Cat 6 (due to their age). The 3rd cable I used was an old Cat 5 (not 5e, 8 wires inside) cable. Results with all cables and several speed tests: approx. 7.5 Gbits download, approx. 1.5 Gbits upload. How is this possible? The cable is connected directly to the router at the 10 Gbits port... Even in the browser, I get about 1.5-2 Gbits when downloading, no matter which cable I use. However, if I connect the cable and the adapter to the 1 Gbits port, I get 980 Mbits. Somehow it's all a mystery to me. Or is there a logical explanation for this? Maybe it's the adapter that "boosts" the slow speed (but makes no sense in my eyes)?