It's normal thing how i heard, most smartphone or cell phones which i have (or had) (and it's not only Sony's (earily Ericsson) devices) charged like that. When charger is connecting with device and socket showing 100%, but after remove charger become 99% and ect. You can connect again and will be 100%. The same can be with 99% when is connecting and after remove become 100% 
I never had with that problem in past, so i'm sure you can don't worry too
@Roadrunny76 wrote:
If this is normal, why wasn't it happening from the first day (in my case)? First 6 months I didn't have this behavior. It started about a month ago.
How i noticed it happens rarely and it's not constantly. It's not issues Why you looking for problem in place where is not it?
@Roadrunny76 wrote:
For me it happens every charge for the past month, and this is called reality not looking for problems that don't exist.
List of options what you can do:
1. If is any problem with smartphone functioning one of method to fix it is repair smartphone with Xperia Companion.
2. To contact with local support service.
3. To bring it to authorized service center.
That's general information what you always can do
*This is few options how to fix your problem. But don't forget also and others things which you can try like: to change charger and to see is the same problem, safe mode and ect. But as i siad i don't think that this is issues, coz a lot of devices have the same things. It's hard to say why this happens, but you can try different ways to charge. I always trying some methods to make sure that problem is really is. How i understand you did not nothing to be sure. You can always to do simple things even if you have warranty, it's to bring to center to test your battery and smartphone. If will be problem - center will fix it
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