the matchmaker often matches you against players with higher individual ranks to compensate for the perceived advantage of having superior communication/teamwork from being in a group. The vast majority of players who play competitive never group with more than one friend...and in solo queue or duo matches, the game plays completely different.

That's the gist of it anyway...pretty much every Overwatch content creator on youtube (Your Overwatch, Unit Lost, etc) has made videos breaking the topic down further.