Unlike Hebrew or Greek, the Roman alphabet carries no built in numeral values or letter by letter symbolic doctrine of its own; Roman numerals are a wholly separate system of seven symbols, I, V, X, L, C, D and M, layered onto Latin rather than derived from the ordinary alphabet's letters in sequence. Esoteric traditions that want to work with the Roman alphabet numerically, the Pythagorean and Chaldean numerology systems among them, therefore have to invent a value assignment from scratch, which is exactly what those two modern systems do, each in its own incompatible way.
The alphabet's esoteric significance in Western magical practice comes almost entirely through what has been written in it and what has been substituted for it, rather than through any property of the letters themselves: grimoire traditions from the fifteenth century onward built substitution ciphers, Theban, Celestial, Malachim and Passing the River among them, that stand in one for one for these same twenty-six letters, treating the plain Roman alphabet as the hidden message underneath a more mysterious looking surface rather than as mysterious in its own right.