Taylor, or it's some sort of other "fouling". If not wiping to the breech-plug every shot or not wiping at all, there will be fouling buildup somewhat in the 'chamber' area. This is simply normal.
If your rod is marked for your standard load in a clean barrel and that gun is shot "all day" without wiping, say 20 to 100 shots, you will find that the mark on the rod is above the bore when the ball is seated with the same pressure, each and every time. As the shooting progresses, the mark gets higher and higher above the bore. If you MAKE the mark stay at the muzzle's crown no matter what, you will have to be over-compressing the powder to make up for the buildup in the breech - don't do that. Your load, to be accurate, must be loaded exactly the same, each time- remember- consistency.
In the 1800's throwing the rod onto the powder or compressing the charge too hard, was called "crushing or mealing" the powder. This caused different pressures and velocities which caused and causes fliers.
If you wipe the bore down into the breech area, the rod's load mark will remain level with the crown - obviously, as the powder chamber has never changed shot to shot. Without wiping, that 30gr. or 80gr. or even165gr. of powder will leave some buildup in the breech, every shot, which over time (long or short) will reduce the powder chamber's diameter, and thus your stricken charge will take up more room. On targets, I have never been able to prove to myself that this has ANY effect on accuracy as the last group of the day is virtually identical to the first ones.
By the end of the day, it is common to see 1/4" between the mark and the muzzle's crown. This matters not, if the load is seated exactly the same, with the same pressure on the powder, the accuracy will be unchanged.
When cleaning the rifle in the little stainless bucket (I stopped using coffee cans or Folgers plastic can a few weeks ago) the water only turns somewhat grayish as the only fouling that is in or on the gun is not very much at all - what's on the lock, outside of the breech, inside in the powder 'chamber' area and what is left in the bore from the last shot will only turn the water grayish.
Just for the heck of it, I've found if I load 20gr. of powder for my last shot of the day - puuuuut- ------------tunk on the big #1 target plate - 20gr., which barely fills the hollow of the breech plug in the .69's patent breach - most ALL of that accumulated fouling form the day's shooting has now been wiped from that chamber's walls and pushed from the bottom of the grooves back to the plug itself by the patches ball - then fired.
The cleaning water of that day's shooting which had that 20gr. 'cleaning charge' fired, barely changes water colour at all (includes outside barrel breech fouling and lock fouling) as all of the builtup fouling that was in the breach was blown out with the last 20gr. shot that made barely more noise than my .22 cal. PCP air rifle.
