What if you're on some obscure lesser known mobile network and the EU country you're traveling to doesn't support that network? Vodafone (a non obscure network) however seems to be ubiquitous in the EU.
It may go even further - the brand name of the network you recognize may be not owned by the same company, as the rest for the networks with the same name. For example, O2 CZ and O2 SK are not owned by Telefonica (anymore), but the other O2 networks are.
When there's no network with the same prefix, the choice is usually random. It doesn't matter anyway, money-wise it is the same.