Flavor variation within a single batch

I have been brewing for a year and a half with 7 extract batches and 8 all grain batches so far. When I bottle my beer I put the last, usually partial bottle of beer in my pantry to drink as the first beer of that batch.

In the last three batches that I have made, I've noticed that the bottle I set aside has looked or tasted different that other bottles from the same batch. Coincidentally I just started using gelatin after fermentation and dry hopping. I mix the dissolved priming sugar into a bottling bucket. Could I be getting a "settling" affect in the bottling bucket that makes the last bottle or few different? The last bottle is not representative of the batch.

Any ideas?

Thanks in advance.

 

My two thoughts:

1 - How are you mixing in the priming sugar? The preferred method is to dissolve it in boiling water and then pour it into the bottling bucket first, then siphon your beer on top of it. If the siphon hose is coiled down at the bottom of the bottling bucket, it will create a slow mixing effect as the beer enters the bucket, mixing in your priming sugar evenly. If you are siphoning your beer first, then dumping in the dissolved priming sugar, I could definitely see some uneven mixing happening.

2 - Is it only that last, partially filled bottle, that tastes different? If so, my next thought would be oxidation due to the extra headspace compared to the rest of the bottles.