From the couple of things I've looked at, I believe the site is somewhat discounting the general massive swing to Labour in the polls. For example it is suggesting many Green tactical votes in seats where I think the Greens could be swept aside by a general Labour surge. We shall see!
The voting system is hideous, but overall it is the easiest, cheapest and most decisive method of voting so I have learned to pinch my nose and bear it....but anyone mention the word 'mandate' and I can still be triggered...