Waveforms aren’t actually mirrored.
They have a plus and a minus side. Imagine the center-line of the waveform you posted is ZERO. This is absolute silence.
Now imagine anything above that line is plus (positive), and anything below that line is minus (negative). The best analogy to this would be to imagine a speaker cone. Picture the cone sitting idle with no sound being produced. This can be equal to the ZERO point of the waveform.
Next picture a super super slow waveform developing: as the waveform starts to rise above the zero-line the speaker starts to push outwards. Conversely when the waveform starts its travel back down through the zero-line into the MINUS half, the speaker would pull inward. And neither the PLUS nor the MINUS are mirrors of each other. It is possible for the speaker to push out farther than it does pull back. And vice versa.
You can also get a rectified waveform, which is everything below the zero-line chopped off!
So they’re not mirrored at all. And I’m sure if you look at your sample really closely, you’ll find minuscule differences from the positive to the negative side!