Voiceover waveform uneven

Waveform assymetry is perfectly normal, although generally it's the other way around from yours - greater positive peaks. When you speak, you project air forwards and that will temporarily displace the microphone diaphragm in a forward direction away from the speaker - and by convention, that's a positive-going waveform. It can often get worse with things like brass instruments, but there is one thing you could try which should at least reduce the effect...

Because the NT1-A is a side-address mic, it's worth getting a couple of elastic bands and a pencil, and temporarily mounting the pencil vertically in front of the capsule. Yes I realise that this sounds mad, but actually it isn't; the air moving forward from your mouth as you breathe will hit the pencil before hitting the diaphragm, and be deflected away sideways. This is also a good cure for excess mic blast or 'popping'. Even a normal pop filter will reduce the effect.

None of this will make it go away completely though, as the air also affects the way your vocal cords work, in a very similar manner - so some of the effect will always be there.