Not Updated For Current Season
The masteries shown here are not yet updated for the current season, the guide author needs to set up the new masteries. As such, they will be different than the masteries you see in-game.
This build suggests health/mana regen at the start, then movement, followed by good health and ap items which complement survivability and good damage output with his 3 ap abilites (box, dagger, 'mirror' ult). Basically use his dagger to harass and do great damage, thus lvl it up first. Then lvl up box and stacks heaps of them in the brush/jungle to get some easy kills. Anyway hope u like my take on an ap shaco.
GL HF ALL! :)