The hopes are to find a block with far less than the amount of coins deposited, then you'd receive the majority of your deposit back along with your share of the rewards from the block found. Also as far as myself putting in 93%, that is just an example. If noone else joins me in this venture then I am contributing 100%, but if another 29.5 coins are deposited in addition to the 29.5 that I'm depositing, then I would be contributing 50% of the 59 coins.

The reason for wanting more than 25 coins to start with is what if it takes 26 coins to find the block? If you only had 25 deposited then you would have run out before a block was found, whereas if you had one more coin, then you would have a loss of 1btc rather than 25.

Another example, let's say you had 25 and it took 49 cause you had a very bad streak of luck. Your out 25, but if you had 50 deposited and it took 49, then everyone would lose approximately half of what they invested, which is far better than all of it.

Hope it makes more sense now

Another example... Watching another group recently, I believe they started with deposits around 18 or so coins, I kept an eye on it constantly. It was literally less than a half hour or so that they ran out of coins that they found a block. Imagine if it had taken just a little longer and an extra 0.5 BTC would have made the difference between losing all of your investment rather than making a return on it.

One more thing... When you refer to less than 1PH  as if it's not worth trying, I would like to say that I've joined willies solo mining group from Germany twice now hashing with 500TH, the first time we found 3 blocks with 5 coins, and currently we have already found one block with 500 TH and have a few days left from combined deposits of 16.4BTC

So it is possible