The speech recognition engine alters the microphone volume based on the volume it detected during the microphone setup. This is not VoiceAttack-specific.
The "Prevent Speech Engine From Changing Microphone Volume" option on the "System / Advanced" tab of the VoiceAttack options window can disable that behavior.
Note that this changes a Windows setting that will affect the speech recognition engine in any applications that utilize it.
Also note that for speech recognition, specifically, 100% volume is not necessary the best setting.