Well it all depends, there are a lot of factors on turn 4 which could cause variances.

Phase 1 = Spiders

Phase 2 = 2 soldiers 2 knights. If the raid is dpsing the Soldier on the other tank, when its stone skin breaks it will not be able to attack you for a bit which ultimately causes him to take less damage. He could also be kiting a little, or popping a short cool down which helps a little and has a very low reuse time.

Phase 3 = 1 dreadnaught 4 spiders ( which get eaten ) Only one of you is tanking. However if it appears you take more damage tanking this mob then the OT it could have to do with you not popping cool downs, or virus not being on the dread.

Phase 4 = 2 rooks, 4-5 spiders. If you are both tanking a rook you are both taking equal damage, however if you aoe threat rotation is better than the other tanks then you will be tanking more spiders than he is which means you are taking more damage because he is doing less.