On X10SLL-F and X10SLH-F, we have noticed the onboard USB header only provides one USB3 link.
Is this normal? and why?
Yes. The X10SLL-F for example has an onboard USB3 controller which supports only 2 USB3 links.
The mainboard physically has 3 internal USB ports.
The "type A" internal connector uses one USB3 link; the dual port internal USB header can only use the last available USB3 link meaning the dual port header supplies only one USB3 port link.
This is also mentioned on the website "2x USB 3.0 (1 Type-A + 1 via header)"