Someone can correct me if I'm wrong, but the data line (top) may also have carrier aggregation (connecting to multiple towers/LTE bands) and I think when that happens, it shows the bars for the weakest signal that's part of CA.
AT&T is also known to mix and match a lot of different LTE bands (2, 4, 5, 12/17, 14, 29, 30, 66), so the chance of being on a different tower or panel can be very likely.