In case of Optimal or LRU when i increase the no. of frames then the no. of page faults will either decrease or remains same. But in case of FIFO sometimes even if i increase the no. of frames it is not going to decrease the page faults and infact it is going to increase them. That's why FIFO is having Belady's anamoly, as FIFO doesn't have a property called Stack property and it's not a stack algorithm.