I would suspect that your browser isn't really opening all three tabs simultaneously. As soon as you connect to TFL, your computer receives the data which display the three sub-forums. At approximately the same time, TFL sets a "last visited" cookie on your computer.
The first time you tried it, the cookie was set just before your last window loaded.
The second time you tried it, the cookie wasn't set until after all three had loaded.
The reason the results are unpredictable is due to internet latency, the load on the server and the speed of your computer in receiving the packets of information sent from TFL's server to your home computer. Sometime's the "last visited" cookie will be set after the first tab loads, sometimes after the second and sometimes after they all load.
I don't think there's any way to control it.
Hope this helps.
-Dave