I think that it's simply a matter of wear. Take a gun that's been shot for 30 or 40 years and it's going to feel very very smooth because all of the interference conditions will have been worn away. Carry up will also most likely come up a touch short due to the wear on the hand and the drive lugs on the extractor.

I have a 610-3 with the internal lock and some MIM internals with either a forged hammer and trigger or color case hardened MIM hammer and trigger. The only thing that I've done to the action is install a 14 lbs. rebound spring and tweak the strain screw to a 9 lbs. DA pull. On all six positions of the cylinder it feels like I've installed needle and trust bearings on hammer and trigger.

On the other hand my 620 was a bit "stagy" when I first started tuning on it. It had a definate "hitch" in the action after the cylinder stop dropped. One cause was the trigger was dragging on the frame. Installing a 0.0015 shim took care of that issue. However, while testing the action with the hammer out I found that 2 of the drive lugs in the extractor were just a bit "tight". A few passes with a 1200 grit diamond lap resolved that. Now it runs as smoothly as my 610 and I know for a fact that the hammer and trigger are MIM. However, just shooting the gun for 10 or 20 years would have caused enough wear to smooth it out, all I really did was speed up the process a bit with the application of a touch of clearance in the right spot and a few strokes with an abrasive that cuts very rapidly.

Point is, sometimes you get lucky and everything fits just right, other times it takes a bit of fitting to get it right and that can be done either by a lot of use or some careful application of a stone.

PS; I also have an older 67-1 that does come up a touch short on carry up when trigger or thumb cocked in slow motion. That doesn't bother me a bit because that isn't how I shoot it, I don't like staging a DA trigger. However, it was the inspiration for my tuning activities on my newer guns because when I purchased it used it had a 9 lbs. DA trigger that was so smooth it was a joy to shoot.