Because it isn't a major increase/decrease but it is definitely noticeable. I solo nightfalls easy. I tend to get killed a little more when im fireteams. This isn't solid proof but it is a good indication. Its always been there. Back in y1 when i would solo deathsinger woth gjallarhorn it took no more than 3 rockets. In a team she always had at leaat 1/3 of her health bar after 3 gjallarhorn rockets. Omnigul was a joke to solo in nightfalls but was a pain in fireteams