A video file is a container holding separate streams: at minimum one video stream and one audio stream, each compressed with its own codec. Extraction opens the box, lifts out the audio stream and re-packs it on its own.
The audio inside most videos is AAC at somewhere between 128 and 256 kbps. Flippy encodes your MP3 from that track directly, so you keep everything it had: and, honestly, nothing more. An extracted MP3 cannot sound better than the track it came from.
Planning to edit the audio afterwards? Grab a WAV from the converter instead. Editing a lossy file and re-exporting it lossy stacks two generations of compression; WAV gives your editor a clean intermediate.