Elo is not win/loss but it is related.
1) The amount of change in your Elo is related to which team is predicted to win and by how much ... then by whether you fill those expectations or not.
Actual Result/Expected Result/ Change in Elo
Win/Win/Elo up a little
Win/Loss/Elo up more
Loss/Loss/Elo down a little
Loss/Win/Elo down more
The exact change in Elo depends also on how much you were actually expected to win or lose.
2) A player can not be carried forever. It doesn't work that way. In PUG land there is one common element in every match you play ... that is you. Over some number of matches, it is possible to extract the "signal" value of your average contribution to matches ... that is your Elo. Mixing group and solo matches will break Elo since you will have a different Elo on a team than separately. Honestly, they should never have had one Elo value for group and solo. The other factor they have taken into account is weight class. So you have a different Elo in each weight class ... and it can show ... my Elo in heavies is in a decent place at the moment ... in lights that is another story
Finally, the composition of any given match depends on several factors ..
- the Elo distribution of folks in the queues
- how LONG the MM wants players to wait for a match ... the tighter the Elo match the longer you may have to wait
The actual play of a match is another thing altogether ...
1) Sometimes good players peek around a corner and are alpha'd by the enemy lance that they did not know was there
2) Sometimes good players are tired/intoxicated/moody

and decide that a Steiner scout role would be fun

3) Sometimes good players come across an opponent and decide to duel them

... fun eh? ... until his lancemates come around the corner and leave your team down one
4) Sometimes good players take bad builds (see LRM Dire Whale above) for whatever reason

5) Sometimes in a PUG, good players don't push since they don't want to go first ... and it is a PUG so no one might follow ... so you lose
... and these are just some of the actions that good players might take all the time ... never mind the semi-mythical "bad".
(There are some bad players out there ... mostly I think because they are learning the game ... folks who don't see the indicator showing they are being cored out from behind while they chase the light mech in front of them ... or whose aim is sub-par ... but I think there are far fewer "bads" than good folks doing not so good things ... at least "not so good" in terms of maximizng win probability ... maybe not it terms of having fun.)