As we know, purpose of attackers is finding some high probable differential characteristics in differential cryptanalysis. Then they construct some plaintext structures, plaintext pool and expect some right pairs that satisfy this characteristic with calculated probability. In this context, they determine the number of required right pair then they construct their plaintext- ciphertext pairs pool according to intended number of right pairs.
In other words, If probability of characteristic is equal to p and they need k right pairs for some key recovery process then their plaintext-ciphertext pools consist of $k*p^{-1}$ elements.
My question is, why they always need some integer number right pairs? For example, I was looking The Rectangle Attack – Rectangling the Serpent and in this article they target 8 right pairs then according to characteristic, they use $2^{72.8}$ pairs to obtain them.
In this example, they may use $2^{73}$ pairs and expect $2^{3.2}$ right pairs for key recovery. Is there any reason to pick expected number of right pairs as integer? Is it always optimal?