Many models will be nose heavy or tail heavy depending on what equipment is installed. Maybe the prototype used a lightweight bushing engine, and the reviewer who experienced a nose heavy condition used a ball bearing engine (which is heavier). Sometimes you can move the battery pack forward or backward enough to balance the model without adding any weight. If not, adding a little weight to either end of most 40 size trainers is not the end of the earth.

I suspect: If you bought ten 'identical' ARFs and assembled all of them with 'identical' parts, each one would require something slightly different to get the balance just right.