Bukkit/CraftBukkit — comment on commit a1b86a1fe14c7689d4e799033636bcd87852e199
File: src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java line 553
By Dinnerbone:

Because it implements OfflinePlayer which has a method to retrieve the current online Player object, if it exists. If it's a Player already... it exists and it's this object.