Xfinity Mobile data usage doesn't match iPhone Cellular data statistics

For the past two months my data usage stats on the Xfinity app/website say I'm using more than 16 GB of data but on my iPhone Cellular settings it shows 7.9 GB of data used. I reset the stats on August 4 the same day my new billing period started. I called Xfinity support but unfortunately the rep wasn't helpful. I also work from home so I am mostly on wifi. I've seen many posts similar to this on the community forums but there doesn't appear to be any solutions. Any ideas of what to do?