Altbier update: a mystery

Today (nine days after brewing) I decided to take a gravity reading of my Altbier and taste the fruits of my labor. To my disappointment I found the 5-gallon sample to be a bit phenolic with what I think is some diacetyl. However, the gravity reading showed it's only at 1020 (O.G. was 1050, and predicted F.G. is 1013), so I'm hoping that the remaining fermentation will clean up some of the off-flavors. But what's weird is that I also took a reading from the 6.5-gallon carboy, and it was at 1010. It also had a much cleaner taste. Now, as far as I can tell there are four main differences between the two carboys:

1. The 5-gallon carboy may have been closer to 80° than 75° when I pitched, as I was trying to cool 10 gallons with an immersion chiller on a hot day (by the time I finished with the 5-gallon carboy, the chiller obviously had less volume to chill so the temp was a little lower).

2. The 5-gallon carboy had less head space.

3. The 6.5-gallon carboy is actually lying diagonally in the chest freezer (it's the only way I can fit two carboys in my chest freezer).

4. The 5-gallon carboy went without a fermentation lock for half a night.

So my question is: why is the 5-gallon carboy is 10 gravity points higher? And will it finish if I give it another week? Obviously I'll know the answer to the second question in a week, but I'm just hoping I don't have to dump five gallons of brew. Stay tuned for the exciting conclusion of "As the Hydrometer Turns..."