I just realised that 666 is DCLXVI in Roman numerals (all numerals in sequence from largest to smallest). I have checked wiki but there was nothing mentioned about it.
Are there any theories or explanations for this?
Actually no one knows what '666' stands for. There is no evidence that it has some connection to Roman numeral, but not, say, Greek (or Jewish) numeral. Thus there are dozens of guesses, but no single answer.
Some examples to produce '666' are:
Nero: N+R+O+N + Q+S+R in Jewish numerals
Domitianus: A. KAI. ΔOMET. ΣEB. ΓE. i.e. "Imperator Caesar Domitianus Augustus Germanicus" in Greek
Pope: V+I+C[AR]+I+V[S] + [F]I+L+I+I + D[E]+I in Latin
Latin: Λ+Α+Τ+Ε+Ι+Ν+Ο+Σ in Greek
King of Israel: המלך לישראל
Another point is that it may be not about numbers at all. For example, in 2 Chronicles, 9, 13 is written:
The weight of the gold that Solomon received yearly was 666 talents
So there is a guess that '666' is a metonymy for "gold" or "money".
Yet no one mentions anything about 'DCLXVI'.