The convention that smaller number written to the left of bigger numbers should be subtracted instead of added, is a later addition to the Roman numerals.

The earlier system would write VIIII instead of IX.

With the original Roman numerals, the order of writing was completely irrelevant, because all parts were added and addition is comutative, so VIIII is the same as IIIIV or IIIVI.

Even in the later variant of Roman numerals, you can change the order of many symbols without changing the value.