Originally, there was no consistency in spelling regarding "numbered" Earths — they would be either spelled out as words or use numbers even within the same story (such as was the case with Earth-Three in 1964) but a tradition of spelling them out developed. Because the current Multiverse (brought back via Infinite Crisis and 52) uses numbers some people mistakenly believe that this was a way DC separates the current multiverse from the "original" one.