One IP address, two different countries [closed]

The simple reason is of course that a public IP-address is a internet network address and not a street address with a fixed geographical location.

An simple IP-address to Location database is similar to using the logic of an old-school landline telephone number (which was somewhat geographically fixed with a country code, area code and local subscriber number) in the era of cellular phones:
although a cellular phone's number looks similar to any other phone number you can't apply that same logic anymore. You can't reliably determine the phone's location from just the number because it's owner can be reached and calling from nearly anywhere in the world.

You can still infer some information and make somewhat reliable broad assumptions from just a phone number. For instance: most mobile subscribers with a french country code are likely to actually be in France, rather than travelling abroad...

Returning to the topic of IP-address to location database: not all of them are updated purely with IP networking information from the same public sources: they can augment their database with additional (privileged) information sources such as actual routing tables, collected meta data about IP spaces but also for instance use aggregated HTML 5 geolocation data for greater accuracy.

Different geolocation sites pull their location data from different databases. There are a lot of these databases and IP addresses get reassigned frequently so it can be difficult to keep the current location of an IP address owner up-to-date in every database all the time.