I'm gonna explain with a simple example, in which there are only two miners: a slow one and a fast one. Both mine continuously at a constant hashrate.

Every time someone submits a share, that share is added to the shelf. The 'value' of such share is 25 BTC/bitcoin_difficulty. That is totally independent of the speed of the miners.

The share shelf contain mostly shares of the 'fast' miner, with the slow miners shares more or less uniformly dispersed trough it. The relative proportion of the fast vs slow shares in the shelf is the same as the relative proportion of their hashrate, and thus the same as the relative proportion of their fair reward.

Every time the pool finds a block, it will pay the topmost 25 BTC worth of shares of the shelf. As stated earlier, since the shares are uniformly distributed and in a fair proportion between the miner, the payed amount to each miner is fair too.

The difference between a short and a fast round is that in a long round, all miners will receive for that round an amount less than what would be expected just by the number of shares they submitted (and the shelf grows), whereas for a short round, miners will receive more that what would be expected just by the number of shares they submitted (and the shelf shrinks). However each round pays exactly the same amount, and that payout is fair, because it is in the same proportion of their respective hashrate.