There can be lot of factors which can be possible reason to this:

1. Technical Reason ; The platinum product may have different level of underwriting as compared with Gold + Super top up. I’m not sure which sum insured you are looking at, However, for higher sum insured in some products companies do a medical underwriting and thus pooling a better risk. On the other hand, Super top up may or may not have a strict underwriting, hence the company may need to subsidise the cost of poor underwriting by higher price.
2. The product combination could be different. Both Gold and Super top up may have their own no claim bonus which may increase SI faster than Platinum plan.