I don't know how flat pitches were back then, you know better, since your older, and watched matches from back then. But I'm pretty sure, the pitches today are much flater. Afridi faced different kinds of bowling attacks, in his first 28 innings, he faced the likes of, Vaas, Jayasuriya, Muralitharan, Walsh, Ambrose, Bishop, Mcgrath and Warne. Those are the bowlers Afridi had to face in his first 28 innings, and thats why he had that 22 average. He still had the higher strike rate, and more 50's than Yusuf, and that was in the "bowlers era".
Yusuf on the other hand, made his debut in the "batting era". In his first 28 innings faced bowlers like, Kyle Mills, Vettori, Ian O'Brien, Steyn, Parnell, Sohail Tanvir, Abdur Rauf.
The bowlers that Yusuf faced against are much poorer compared to the likes of, Walsh, Ambrose, Mcgrath, Vaas, Murali..
Yusufs average would 11, if it weren't for the Not outs...The only reason his average went up from 22 or 21 I believe? Was because of todays 123..Not Out
So, even when facing easier bowlers, he still isn't better then Afridi