Good Lord: I wore out a 103 bottom end!!

[S-Class Guru]

How could this have happened? I have "only" 240k very pampered miles on my '91 300SE. She should have gone 300k at least!
Here's the story, and comments welcomed.

A couple years ago, static compression dropped a little on #1 only, and continued down to 155 last year. About that time I pulled the head to fix the infamous gasket oil leak. So, I had the head checked, and I looked over the cylinder bore, seeing zero evidence of any problem. No oil usage, clean plug, normal piston top carbon, etc. I bolted her back up and hoped for the best.

Well, she just slowly went on down to 105!!!! All others are 170-180.
I shot oil in the cylinder, and she popped to 200, indicating ring leakage.
I shot compressed air in, and could easily hear the hissing thru the crankcase (actually blew oil out the dipstick tube when I held my finger over the PCV tube).

So, gotta be rings/piston/cylinder - right?

I pulled the head (had it checked again) pulled the pan, yanked the piston, and could see NOTHING wrong! I did not have a bore mic, though, so just judged the bore by the mostly still-visible hatch marks and the total lack of a ridge at the top. Piston looked virtually new after soaking. So, I made the decision to brush-hone the bore, install new rings in the cylinder, and hope for the best. The only other option would be to pull the block and have it bored - ouch!

No luck - buttoned her back up and ran her 500 miles - 105 PSI - AAAAARG!

So, what am I missing here? After having the head professionally checked twice, and doing the dry/wet compression tests and homemade leak-down test, I'm convinced it's ring/piston leakage. The brush-hone quickly cleaned the bore up perfectly, piston looked virtually new, and the new rings fit well, and had good end-gap in the cylinder.
So, I'm assuming I must have taper or "egg shape" wear in the bore, enough to inhibit ring seal?
I do remember the #1 plug being a bit blacker than the others a few years back, when I was on my 2nd set of injectors. I had some starting & emissions issues and eventually replaced the injectors last spring - I wonder if I had a leaky one on #1 and it washed the cylinder wall for a few years, causing abnormal wear?

I'm on my way down the road to a complete rebuild, including boring all cylinders, but just thought I better run this by you guys before I jump all the way in and spend a wad of $$$ and a ton of hours.

DG