Aditya Birla - Activ Fit Plus (10L+10L) with 90L Niva Bupa topup

Hi, My age is 25Yrs with no medical history (Non-smoke and drink). I compared the Care supreme (7L) having super NCB top-ups and Heath test top-ups with Aditya Birla - Activ Fit Plus (10L+10L) with 90L Niva Bupa top-up.

The latter seems cheaper and more beneficial to me but still trying to understand why a top-up of 90L is costing only 851 extra whereas adding a small amount in base costs much higher. Is it something I am missing here?

Also, is there anything I should be aware of in terms of buying Primary insurance from Aditya Birla and top-up from Niva Bupa? In case of claims exceeding the base, will it be any hassle since both are different companies (Cashless etc)