You are not the only one. Mine says the same - 100%. This is a bug. The terminal command for querying the battery percentage (probably what iStat Pro uses) reports: "BatteryPercent" = 82
ioreg -n "BNBMouseDevice" | grep -i "batterypercent"
I've posted this in the Apple Bug reporter. Hopefully it will be fixed in Lion 10.7.1
Here is another thread discussing the same issue. https://discussions.apple.com/thread/3211868