Because you are paid in BTC and we mine different coins which need to be exchanged into BTC first. Last 24h profits is your share of what the pool mined in these 24 hours. Payouts are sending you already exchanged BTC. Coins mined in day X are paid mostly in days X+1 and X+2 and them some much lower amounts in later days depending on how quick is the coin to mature and exchange.
So basically it is the difference in the fluctuation of the prices of the coins from the time they are mined until the time they mature and are payed out that causes the difference in numbers?