I find this very interesting. I can come up with two theories.
Theory #1: On full through swings, you have a tendency to slide your hips too far towards the target, and don't stay behind the ball at impact. The result is a blocked shot that starts right and stays there, sometimes with a little fade. The right hand swing made it nearly impossible to slide your hips, so you're turning them properly instead, starting the ball on the intended line.
Theory #2: On full swings, you tend to take the club back too far inside, causing a severe in-to-out path on your swing, and causing a push. On right handed swings, you're taking it back on a straighter path, causing a more over the top swing, reducing the push or causing a pull. You said that you bring the club back "straight" and to about "12 o clock" so this would make sense.
There are numerous drills to address the above swing faults. That should give you an idea on what to work on. But without seeing the swing, it's hard to tell. Just hoping I could help!