It's because the monitors are different, they are technically different models. Specifications could be identical but they can still vary. I've got the same issue with 2 Acer monitors, every feature is identical but the color varies a little and they are different models. I just tweaked the settings on the monitors themselves until the color matched close enough.
The monitors are different. Additionally, there are many TN panel manufacturers that have a slightly different manufacturing process that results in some minor differences. LCD monitor uses panel from several sources.
Specs are are basically generic.
Set up one monitor to what you like, then try and match it with the other monitor.