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Without more info, here are a couple of questions to consider:
1) Did you allow the spring to compress/extend during twisting? Not allowing this would stiffen it.
2) Are you confident in the properties? I've found that drawn wire for springs may not exhibit the same elastic properties as the base material. It was quite a shock the first time this came up in a project. Tensile testing on your spring wire is the only way to check this.
3) Are you confident in your restraints on the fixed end? While it probably wouldn't account for all of your rotation, any slippage in the test fixture restraint could exxagerate the empirical measurements.
After these, I'd need to see some pix of the test set up and your model.