I had my usage suddenly go from 300GB to over 1 TB (the cap) and I couldn’t figure out where it was coming from. This was after months of my partner and I both

I had my usage suddenly go from 300GB to over 1 TB (the cap) and I couldn’t figure out where it was coming from. This was after months of my partner and I both working from home and keeping the same usage habits. I tracked the usage on my router and it didn’t match up. I asked the ISP for proof and they had none. Fortunately I found fiber had just gotten to my house and had no cap so I was able to switch, but I wondered how much you can really trust the ISP accounting.