The old Myanmar fonts did not enforce any particular order on vowels. It was therefore common to have ◌ိ and ◌ု stored in different orders in a word like ကို depending on the typist. There are also many variations on which medial form is used in words like ကြိုး. There is usually one form which is prefereable, but typists are sometimes lazy and type a suboptimal form. In Unicode, only one order is correct and the font substitutes the correct glyph automatically.
This means that if you want to convert from an old byte encoding to Unicode, you need to consider normalisation and reordering as well as straight substitution.