I tried to make a point trying to explain the same.
Its not always mathematical, majority of the times it is sentiment driven - just like the stock market.
Market doesn’t like uncertainty.
With holding cos the main issue is uncertainty.
You are valuing the company by underlying asset & trying to correlate that into the price.
But we often forget that the value gets only realised if they are to sale their stake & than distribute it to Preference shareholders & ordinary shareholders.
The uncertainty part is , no one knows when the value unlock would happen and therefore the holding cos discount.
There are other factors which can be added, but the above one is the major for such discounts.