Because this is just mathematical cargo-cult scientism. If the basis for deciding "the model itself may require tweaking or redesign" is that "you still think the results look wrong", you're not doing science.
If you're modeling investment growth and your model is:
future_value = investment_amt * (interest_rate ^ num_years)
future_value = investment_amt * (1 + (interest_rate * num_years))
future_value = investment_amt * ((1 + interest_rate) ^ num_years)