It is because the restriction by the provider of the sportsbook IMO, while the casino is using their own terms so the restriction can be different.
That’s correct.
Every game provider and sportsbook has its own terms, which might differ from the casino’s terms. So, if your country is allowed by the casino, that doesn’t necessarily mean it will be allowed by the sportsbook or some of the game providers they partner with.
Although a casino claim to be VPN friendly, it does not mean that players are allowed to change their IP to bypass the restrictions by the terms.
VPN friendly should be taken as if we are from non restricted countries but we want to use VPN for more security/privacy purpose only.