turning beer into water/some bottles are watery some dark

I brewed a batch of Oatmeal Stout my Sister-in-law gave me for Christmas. It had the run-of-the-mill 3.3 dark lme and 3.3 light lme and some grain to steep. I didn't properly journal this one but If I remember correctly I did a:

-5.5 gal batch
-6.6 total lbs lme
-partial mash with a wee bit of chocolate malt,
-2lb marris otter and
-1/4lb crystal and
-a total of 3lbs oatmeal
-Wheat dme boiled and mixed into secondary on bottling day.

Some of my bottles taste like tea and are light brown while a few are dark but watery. I think I had one that was about the right strength soon after I bottled. All the sugars are fermented because it doesn't taste sweet. There was plenty of stuff on the bottom of the primary which is evidence that there were enough ingredients and my target abv if I remember was supposed to be between 5-6% (I don't have my notes). I know I'm not getting my batches mixed up because it was oak aged.

The only thing I can think would be the cause is if I mixed water in after fermentation to increase the volume and didn't mix it well enough.

Has anyone had this happen? Any ideas what could possibly cause this?

Thanks!

John

-AG John's Something Delicious-conditioning
-Oak Aged Oatmeal Stout Partial Mash Christmas Present- bottled and in question
-AG Great Divide Oak Aged Yetti clone- tragically one fantastic bottle left!
-End of Summer APA-gone
-Pumkin Ale- Could have been alot better, it's hard to drink and harder to get rid of!

I'd bet not mixing enough was the culprit.. I've never done an extract batch or topped up but I've heard it takes ALOT more mixing than you'd think..