I believe your comparison is misleading for this event.
The Tiger's versatility and high dpm is what enables it to score very well in tier 7 matches; better than the IS. The Tiger 131 even more so.
You may well be right that the IS is a better tank on many measures but the Tiger is about the best tank to get consistently high xp in tier 7 matches. They are great for feasting on tier 5s and 6s and can still smack tier 7s. The IS is great but it has a slower reload and its fantastic alpha is often wasted on finishing a half dead tank for a hundred hp or so.
Play a few matches at tier 7 (tier 7 as top tier) and you will observe that Tigers (incl Hammers, 131s an Ps) are prevalent at the top of the board on each side - IS not so much.
That is why I think that over the weekend Tigers will dominate in tier 7 and that it will be a significant contribution to the overall result.
Tigers fare much better when bottom tier as well.