Russian clubs are well scouted I'm just mentioning that if he was that talented he would have been spotted quicker. A scout at Barca went and seen Messi and after 10 minutes of watching him play ( when Messi was 13 ) he decided to sign him. Good players get spotted before they are 25.
He just developed late, which is the primary explanation for him going unnoticed for the early part of his footballing career. And to add to that, he was played in various positions until he was finally given his favourite position at Zenit - so he probably had no rhythm and couldn't hit his stride until he matured a bit. There are exceptions to the notion that "good players get spotted before they are 25" and Arshavin is one of them. Each footballer is different and perhaps because of a series of factors, something happened to 'click' and Arshavin became who he was. Scouting is also different in South America and Russia. I am sure that most clubs have more scouts operating in nice and warm great player breeding-ground South America than in cold and sometimes unwelcoming Russia.
In the end, Arshavin won't become a world number 1, he won't be up there with the Messis and such. Or at least it seems utterly against the odds. But that doesn't mean he can't be a world-class player...