I assume they are coming from a different IP address from the one you've configured in the data source configuration of your cisco router.
Look into logging source-interface XXX on your router if you have multiple interfaces.
Also, another way to do this is to enable Auto Learn data sources.
Regarding netflow you will have to be more specific. netflow does not replace syslog in terms of information but you can have syslog + netflow if you use 2 different IP addresses.