There is no official 6.5-284Win. That’s a wildcat obtained by necking down a 284Win and dimensions could theoretically vary from rifle to rifle since they’re all custom, and chambers were reamed with non-standardized reamers. Throat length will probably be the most variable dimension.
The 6.5-284Norma was standardized. It was based on necking down a 284Win. If you have a SAAMI spec 6.5-284Norma then you know what you have.
In practice there isn’t much difference, but the 6.5-284Norma has a long throat and if you have a wildcat 6.5-284 some factory ammo may not chamber. Accuracy with factory ammo will likely be much better through a 6.5-284Norma as it was designed with similar accuracy improving traits to the 6.5CM. Shooting that ammo through a chamber that doesn’t match will not always yield equal results. A good 6.5-284Norma will shoot EXCELLENT groups with quality factory ammo, just as 6.5CM will. For hunting the 6.5-284Norma is a 6.5CM on steroids. It’s everything the 6.5CM is except it’s faster.