The issue is that proving why a juror voted a certain way is kind of tough. The beautiful part is that this is a feature, not a bug.
Its interesting you meant that rhetorically, your premise is wrong in my opinion. If you want to distinguish between an instruction and an order I'd think it has to be based on how it can be enforced. An instruction can't be enforced by a judge, for example they can instruct you to only consider what was presented but they can't punish you if you disobey that.
Judges will be careful not to directly give instructions against nullification for precisely that reason. They may very well imply that you shouldn't go off the rails of evidence provided or laws and precedent as described, but that's as far as a judge can go with instructions against nullification.
Orders and instructions are different though. An official order by a judge may fall under contempt of court if you don't comply. Instructions are more procedural and about a judge running the process of the trial. For deliberation, that also generally means instructions that really just help guide a jury of people who may not have done it before and aren't sure of the process or general expectations.