Several possibilities.
Main one.
While most PC stuff uses one of a few encodings* then most consoles ever have been quite custom, the DS started to use a few known things (see EUC-JP and shiftJIS.
http://rikai.com/library/kanjitables/kanji_codes.euc.shtml http://rikai.com/library/kanjitables/kanji_codes.sjis.shtml ) for some aspects in some games, and has limited amounts of ASCII at other times.
Others include
Compression. Compression makes things smaller and does it by making things unreadable.
Markup. If you are familiar with HTML then rarely anything that complex but similar ideas. This can also include things like new line indicators.
Possibly unicode type setups -- A through Z for both cases, plus numbers, plus punctuation, possibly plus some Greek letters is way less than 256 options that is the maximum 8 bits can be used to represent. 256 is not even a good start in Japanese Kanji so they jump up to 16 bits (sometimes more) and your basic search might only have done 8 bit searches.
Encryption. Rare on console games but quite common in PC stuff.
*if you ever made a code as a kid along the lines of 01=A, 02=B, 03=C... then same thing for computers except using hexadecimal.
All this is but one reason why "Nintendo DS Translation Editor for any Nintendo DS Games using to make English Translated" will not exist this side of a crazy advanced AI. Even if this did work (though pro tip, try relative search next time
http://www.romhacking.net/utilities/513/ ) you would probably then have said markup to wonder about, pointers and maybe some font issues at the very least.