That's where the difference between participating and non-participating preferences come in. Participating meaning that they also participate in the remaining s

That's where the difference between participating and non-participating preferences come in. Participating meaning that they also participate in the remaining surplus (so on your example $28M). Non-participating means they choose whichever is better (in your example they wouldn't since $20M is better than 10% of $100M, but if the company sold for $300M they'd choose $30M instead of the $20M).