Browsers typically use a few connections to load a page so that it can load faster. Each of those threads has a different source port, and thus may route to a different target. In Colm's demo, it depends which thread your browser uses when requesting the part including the CSS which decorates the object. In my Chrome on Mac I see 6 TCP connections to the demo NLB

tcp4 0 0 192.168.100.101.49615 54.69.111.179.80 ESTABLISHED tcp4 0 0 192.168.100.101.49614 54.69.111.179.80 ESTABLISHED tcp4 0 0 192.168.100.101.49613 54.69.111.179.80 ESTABLISHED tcp4 0 0 192.168.100.101.49612 54.69.111.179.80 ESTABLISHED tcp4 0 0 192.168.100.101.49611 54.69.111.179.80 ESTABLISHED tcp4 0 0 192.168.100.101.49610 54.69.111.179.80 ESTABLISHED

Each of those will be routed to the same target, so it's up to your browser to decide which to use for what.