I'm not an expert, but this is my understanding having asked the question directly to explorers.

Grab samples are always higher ... because.... they cherry pick the rocks they want to sample. For grab samples... this means you're picking rocks that visibly look interesting. In Au properties, many grab samples are picked with visible gold.

Grab samples tell you there is a particular metal in the area. Nothing more should be read into it. Channel samples are slightly better... but you are still cutting a channel through the best part of the vein.

Drill cores are the fairest representation of what is really there, but they also have the highest hit/miss ratio because you can't see what you are drilling ahead of time. That is why so much time is spent trying to delineate the veins before dilling deep.

mz