If you press F3+F4 and you play in English, the text "Next" will be exactly centered. But if you play with a different language (in this case, Italian, because it is a snapshot it's translated with a resource pack), the text will most probably be on the right (In this case, "Successivo" [1] hits the border, while "Succ." [2] is centered, but some languages may not have a four-letters word to translate "Next").