The words in the dictionary file are arranged alphabetically. When your Mac scans to see if the dictionary contains a word, it stops once it reaches the point where it should be. In other words, if you put the word colour at the end of the dictionary, it will not be detected because spell check will only look up to theCs.
Similarly, if you put the word Zebedee at the start of your list, spell check would stop instantly. When you are adding words to the dictionary you must be careful to keep them in alphabetical order.