Pressure sensors in styluses and erasers are subject to wear: after heavy use the resistance strip which measures the pressure exerted on the device may not entirely return to its original shape. Thus its resistance indicates a non-zero pressure even when the device does not touch anything.
The driver is able to compensate for this: It uses the pressure data sent when the device comes into proximity as a bias value which will be subtract from any measured pressure value while in proximity. I will also rescale the remaining range to the full range of the specific device. Therefore the granularity of the pressure readings will get higher. Thus the Wacom Xserver driver will warn when this pressure bias is higher than 20 percent of the maximum value. These warnings will appear in the Xserver logs.
Should you see repeated warning messages about this it is an indication of a worn out pen and you should consider replacing it.
Please note that if you move a device into proximity and onto the tablet very quickly the first event sent from it may also have a non-zero pressure reading. It thus will be taken as a pressure bias by the driver even though the device isn't worn. The driver attempts to compensate for this by adapting the bias value if it sees pressure values lower than the bias coming from the device. It is still possible that you may lose events (ie 'clicks') on button 1.