I think the confusion is between cupro-nickel and cupro-nickel clad steel (CNCS).
As Canam pointed out, the latter is sheet steel with a thin layer of pure cupro-nickel rolled on to it and then formed into bullet envelopes. The magnet is being attracted to the steel, not the cupro-nickel. This will happen even with ordinary lead cored ball bullets.
European manufacturers have used this bullet material ever since jacketed bullets came into use, whilst British (and Commonwealth) manufacturers have used pure cupro-nickel without the steel for their envelopes up until about 1942 when the swicth was made to gilding metal (Copper coloured). Get an old .303 with a cupro-nickel envelope and try the magnet. It will not attract.
Do the same thing with an apparently cupro-nickel European round and it wll attract because of the steel envelope under the CN.
Of course, if the bullet is AP or has a mild steel core then all bets are off because the core will attract. Even then, with a bit of experience it is possible to tell whether it has a core or a steel envelope.
Regards
TonyE