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Well, I have spent most of my forum-going experience over on pojo, but I like the DP tier/moveset pages here. While viewing said pages, I have realized that the EV spreads only go to 508 not 510. Is there a reason for this?
This can be answered in the 'Simple Questions Simple Answers' topic, located on Stark's first page and stickied.
To answer your question: 4 EVs give off 1 stat. Considering that 510 is not divisible by 4 - but 508 is - it's useless having 510 EVs in one Pokemon when the remaining 2 EVs have no effect at all.