Network layer protocol overhead, error correction overhead, other users on network etc.
Many physical interfaces use 8b/10b encoding for dc balance (Ethernet, PCIE, SATA, etc), meaning you've already dropped to 80%. Aside from any spiking, 100mbps Ethernet won't average more than 80mbps, and then other factors can effect from there.
And for similar and other various reasons a local wireless standard like 802.11G 54mbps in best case scenario is around 22mbps. Down 40%.
Then we go wide area wireless standard, even more potential disruptions, needs even more overhead to reliably send packets, it's going to get worse. With that frame of reference, the 3-5mbps I get here in metro Atlanta from my 7.2mbps cell phone is simply stellar.
Expect a 14.4 network interface to potentially double that base, to 6-10mbps realistic.
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Article is total farce, already debunked in other threads. Editor there is clueless.