The Raiders regressed mostly because their QB got injured early and never really got back on track*. There's really no need to make it more complicated than that. It wasn't a crystal ball "single score or less" regression. Carr got hurt in the second half of this team's 4th game. To that point, and counting that game, he had a QB rating of 100 or better in 3 of 4. After his injury, he missed one game, and then only had a QB rating of 100 or better in 2 of his final 11 games, and his team wasn't losing the one score games they'd won the year before, because they weren't even keeping it to within one score. Two of their first 3 losses were by one score (that includes the game Carr had to leave due to injury), but only 1 of the last 7 (6 under Carr) were by one score.
Prior to injury, Carr's QB ratings:
114.3
136.6
52.9
100.0
After his return from injury, he had QB ratings of:
67.5
71.2
68.2
60.1
48.1
making up 5 of his 11 starts. In the team's last 4 games, all losses after the team sat at 6-6, his QB ratings were:
60.1
84.4
48.1
91.8
and his passing yardage totals were
211
171
140
243
In 2016, Carr had only 2 games with a QB rating under 80, and one of those was 79.3, and 9 games with a QB rating over 90. So the consistency was much better, and the level of QB play was higher.
If Carr is 2016 Carr, the Raiders are a team to watch.
If Carr is 217 Carr, the Raiders are a team that will flounder.
*And, interestingly, the Carr situation mirrors what happened with Crabtree and his catch % change before and after his chest injury in week 3, which gives you a 1-2 combo of key offensive players who weren't the same after getting injured in the first 1/4 of the season.