It seems unlikely it’s the cause (probably not set that high), but another possible source of variance is the deadband in Ignition tag numeric properites:
If it’s set as high/higher than the variance you’re seeing, it can cause that kind of variance in the display by not updating the tag value until the source varies from current tag value by more than the deadband amount…
There’s a lot of possible causes for why your values differ. Your system topology isn’t clear for me. Do you have Modbus TCP/IP and RTU and TCP/IP over RTU?.
How the PLC is involved in this topology? Consider the baud rate from modbus and mbps from tcp may be conflictive. This is import for Ignition because the poll rate should be based on that criteria.
Ignoring the above and assuming the data provider is from the same spot. As @witman said,
maybe Ignition it’s been more sensitive to data change compared to other panels, try increacing the deadband. In addition, make sure to create an appropriate scan class for your opc tag subcriptions. Play with this values and you should see the same values maybe with small errors