I don't think it's a faulty battery, rather a bug with .107 I have this issue and it occurs when the phone has been used for a good while be it web browsing, reading windows central, playing a game etc.
Use phone for a good while until it gets slightly warm, soft reboot, power down and you will either have a dead phone, less charge or miraculously more charge than had you before. I believe either the background agent that monitors the battery has a bug or if it's tied into the battery saver app then that is buggy. Here is an example (I noticed it when I was on the grocery run):
Close the battery saver app and go back it's normal:
I haven't been able to replicate this bug as haven't had the time to check it out properly, once I've done so it will be listed in the feedback app.
Furthermore with build .107 after it finished installed my phone turned off, holding the power button to turn on, soft resetting as normal didn't work - I had to hold the volume down + power button for a good minute to get it restart as I was trying to get into the bootloader. But unfortunately the cable I use for flashing has developed a fault so I couldn't reflash so just let proceed with the boot. Which did take much longer than normal - about 90 seconds.
As a side note, my 920 is over 40 months old and I had this issue a few times over span of months with WP8.1 pre-amber and post cyan, about twice post "denim" but on all but few (iirc) occasions I had more charge than before. Therefore it was a non issue for me, so never mentioned it.
With .71 my battery stabilised after a few days and it would actually got decent charge to charge time for a preview build; I'm hoping .107 goes through the same process and stabilises.