Different results between WAVE and FFMPEG reading audio file in Python

Hi everyone,
I've found that if we change the method to read the audio file (from WAVE to FFMPEG), the WER increase dramatically. When I use WAVE to read audio files (4000 utts), the WER is 9.64%. But when I use FFMPEG instead, the WER is decreased to 4.77%. Can you explain why the difference exist?
Btw, I've printed the result of each method when reading audio (_process.stdout.read(4000)_ and _data = wf.readframes(4000)_) and the result was different.