Tabetha here!!!
It is purely down to pressure, ie there is a lot more in the main pipe as this feeds the radiator, than there is in the turbo return pipe, which is slower flow as well so it absorbs more heat, as it stays there longer, thta is why the return from the turbo can be a huge amount hotter!!
It is normal, but shows it is doing it's job as air rises with the flow, so wil expell any air bubbles, these swirl pots are a great idea as some engines suffer from localised boiling(hot spots), which will create air bubbles so this gets rid of them.
tabetha