bandwidth reporting difference from my router

I regularly log my monthly bandwidth consumption using the Xfinity app on my phone. I record the value from the Internet Data Usage tab and calculate the delta from the previous entry.

A sample of the data (trimmed for brevity):

|  Date Range   |  Total Monthly Usage  |  Timestamp Recorded  |  Δ GB  |  Δ Time  |
|---------------|-----------------------|----------------------|--------|----------|
|  01/01–01/31  | 519                   |  01/16 07:09         | 82     |  —       |
|  01/01–01/31  | 530                   |  01/20 05:20         | 11     |  3d 22h  |
|  01/01–01/31  | 575                   |  01/21 06:34         | 45     |  1d 1h   |
|  01/01–01/31  | 675                   |  01/23 07:37         | 100    |  2d 1h   |

For the most recent interval (01/21 → 01/23), the Xfinity app reports 100 GB of usage.

Over that same period, my ASUS router’s Traffic Analyzer reports 40GB of total WAN usage across ~30 devices. I collect per device cumulative counters (upload & download) every 10 minutes and store them in SQLite (600k+ rows), so I can compute precise usage over any interval.

I fully understand that the Xfinity app is not real time and that timing differences will exist. But a discrepancy of 100GB vs. 40 GB over the same two day window seems too large to be explained by reporting lag.

What can I look at to reconcile the difference between Xfinity’s metering and my router’s WAN usage totals?