Nice match making xD

its not so hard to explain actually.

you have 3 gold 1's and 3 plat 5's their MMR are actually quite similar (since plat 5 can tank their mmr) your team had a gold 3 while you were a plat 4. the silver was duoing with one of the goldie and added you into the team because ur plat 5'S were prolly actual gold 3 MMR.

All that being said, that game you were in was pretty close from the look of it (looking at kills, items built, and objs) So the match making actually did a good job there.