You can get the gap betw the frame and side plate look even if the two pices are assembled and polished as one IF the polisher does the job with a loose stiched, soft wheel buffer.
Heavy handed pressure, indifferent directional polishing and usually trying for a high shine, while polishing can push the soft wheel into the joint betw the parts and cause a gap to show.
It's just poor workmanship (Workpeopleship in the 2000's)
Hardback belts & wheels or the old hand methods of polishing with the grit cloth backed up with a block or file while the parts are assembled remove the problem from showing up.
Here's a couple pics of a Ruger OM Blackhawk project
The frame being fitted with a steel gripframe from a Ruger OldArmy' revolver.
The grip frame fitted and held in place with the screws.
Then the assembly was polished as one piece.
If the two parts were worked on separately, a very sloppy final fit and polish would result.
This one roughed out with a belt grinder at this point.
Most every polishing project looks like this if there is some fitting to be done or if the surfaces are rusted, pitted, damaged.
You can see the criss cross approach to working it over to bring both the gun frame and the grip frame down together and flat.
You can still see a very thin sliver of blue showing in 2 places where the gun frame meets the grip frame.
Those are low spots on the frame from the orig Ruger polishing & blue.
The entire surface needed just a touch more flat polishing over all to bring everything down flat.
Once that was done, then those criss-cross belt grinder polishing lines are taken care of by further flat polishing in finer grits.
Again, the direction doesn't matter. In fact it's best to polish across the previous grit you just used. That way you can see when you have completely removed all of those heavier grit marks.
The final grit used will be done in the direction you desire for the final look. That will wipe away the previous grit lines no matter their direction.
If you didn't remove all of those heavier grit marks as you went along, you will have them staring at you through your beautiful polishing job as a heavy deep scar.
The only thing to do then is go back and remove them thru the repeated grit #'s that you used to get to the final gloss you ended up with.
All this is goes the same wether you are polishing a Ruger, S&W, Parker or a Purdey.