NO! This is NOT normal! I had this same issue on GDR2 and upon researching this on the web I found the following helps. \
1) Run you battery down to below 10% (you can even run it down until it shuts itself down).
2) Fully charge the battery using the plug in cable (NOT THE WIRELESS CHARGER!) uninterrupted (best to do this overnight so less likely you would receive a phone call).
This reconditions the battery so it will have a full charge and should fix the problem. It worked for me.
When I upgraded to GDR3 this issue happen again. I ran this senario and it fixed the battery issue again. Battery is working fine, charges to 100% and I typically can go 2 days without charging. Not sure why this happens, but the fix worked for me. Hope it helps you guys out too.
I know this is old, but some clarity on this might help if it comes up for others.
Lithium (ion and polymer, I don't remember which the 928 has, but they're similar) batteries do not have "memory" like NiCad batteries do, so some of the stuff that people did for old rechargeable batteries doesn't apply, and can cause some confusion. For example, the fixes above are to address a SOFTWARE issue, not a problem with the battery.
Lithium batteries lose their ability to hold a charge over time, which can affect the phone's battery display. The phone typically does a decent job of keeping it calibrated (by watching the voltage fluctuations while the phone is in use, or sitting on a charger fully charged), but sometimes it can get thrown off and read incorrectly. Running the battery down until the phone dies is one of the recommended ways to "fix" this (gives the phone a nice large voltage sweep so it knows where 0 and 100% are). This doesn't "recondition" the battery, that's just not possible with how Lithium batteries work so the phone will NEVER hold the same amount of charge as it did when it was brand new. It does, however, get the phone's battery level indicator to be back to being correct, which helps fix situations where the phone doesn't seem to be fully charging, or draining faster than it should. Either way, as your phone gets older, it's battery will appear to drain faster for the simple fact the battery isn't holding as much charge as it used to.
In today's day of upgrading every year or two, most people will never notice the change, but heavy users will likely start to see faster battery drain sooner than average users.
Also, charging wirelessly or with the wire doesn't really matter, the power coming into the phone is the same, the phone doesn't really know the difference, it's just wired chargers are quite a bit faster.