Why does my archgun keep refilling its ammo?

Sometimes when playing with my archgun in regular missions, I find that my ammo (not loaded ammo) gets fully refilled to the amount it had right after deploying the archgun.

I didn't think of it much until I tried using my Kuva Grattler in arbitration. I found that the ammo (in red circle in image) was getting set multiple times to 320 (max amount) before I even managed to use it up. The result was me using the Grattler 40 minutes straight without needing to ever reactivate the archgun.

However, I am not able to point out why or when this happens. It doesn't seem to be happening on a regular basis either.

While this is certainly not an inconvenience, is there a system to this ammo refilling that can be used? Can I refill my ammo without having to wait 5 minutes to re-deploy the archgun?