CPU clock speeds

Why is it that CPU minimum requirements for games often recommend single core CPUs with very different clock speeds? For example, the minimum requirements for Crysis are either a Pentium 4 2.8 GHz or an Athlon 2800+, which is clocked at 1.8 GHz. What’s the difference?

Different processors can execute different numbers of instructions per clock cycle. So comparing the Ghz rating of different processes architectures is a very apples to oranges exercise.