Getting 650MB/sec on USB3 Gen1 ...Really?  How is this possible ?

I have a NUC (D54250WYK1) that I'm using to interface with a custom camera, running Win7.  I developed the USB3.1 Gen1 camera device and was testing the throughput on USB ports of the NUC.  For production (potential product) I would like to have (2) of these custom cameras interface with the NUC.  At my max camera speed, I can send ~275MB/sec to a USB3 port on the NUC.  I was surprised to find out that I could also connect a 2nd custom camera running at the same speed on a 2nd USB3.0 port. I have a tool to provides statistics and sure enough, it shows that 550MB/sec is going through both ports.  To test even further, I have a USB3 drive connected to a 3rd USB3.0 port (back side) and copied a file FROM that hard drive to my SSD drive (~2GB file).  The USB tool tells me I reached speeds of 650MB/sec !  So... obviously there's more than 1 bus instance going on here.  [Which  is exactly what I need !!].

How do I know/specify in future NUC or other Intel MB's, that I need several bus instances of xHCI to guarentee 2 sensors to my MB ?  How do I know which USB3.0 connectors are on seperate xHCI bus instances?

I love the fact that I can stream 650MB through USB devices at once, but I can't explain it, so I have no idea how to specify how to guarentee it.

How many xHCI bus instances are there on this NUK ? How do I find out?

Also, I can slow my camera down to ~175MB and run through a USB3 HUB.  How do I determine the max throughput of the HUB? [i.e. what general overhead would I expect beyond the standard 20% of the 5Gbs?

Thanks!

Mike