this is very odd. There could be more options what is wrong. The devices could report wrong data, caches could have impact to the transmit or something wrong could be in the charts settings.
Check please if you have not additional pollers assigned to the same device.
The best you can do is contact the support. Create diagnostics and if possible include table InterfaceTraffic_Detail and grab at least 30 minutes, rather 1 hour of Wireshark let we can investigate where is the problem.
Upgraded from NPM 10.3 to 10.6 on support recommendation and the history is correcting itself 5 of the 6 show correctly now DB update just hasn't gotten to the data yet from 7 days ago.