The clock displayed on the Today+ screens use the local time settings for the device it is displayed on. So if the device is set up to use network provided time, the Today+ will also use this network provided.
This means that in the situation where multiple Today+ screens are not showing the exact same time, it is most likely due to the devices having different time settings.
A common situation is for an organization to use different devices for displaying Today+ due to a transition from one hardware provider to another (eg. having both iPad and Philips tablets). In this case, if the devices run network provided time, the clocks will not show the exact same time as different devices will have different providers of network provided time.