As "postscript" is one word, why do some abbreviate it as "p.s."?

'Postscript' is a single word in modern English, and Dictionary.com states that it's even based on a single Latin word, postscrīptum. So, why do some abbreviate it to p.s. (or P.S.), as on this Wikipedia page? Is it OK to abbreviate it as ps.?

Update: It seems Dictionary.com may have been wrong with its etymological explanation of the modern English word. Nevertheless, given that the modern English 'postscript' is one single word, shouldn't the abbreviation be able to be updated?

It seems that Dictionary.com might have made a mistake in connecting the two words. Both Etymonline.com and Wikipedia agree on it being separate:

Etymonline.com: 1520s, from L. post scriptum "written after," from neuter pp

Wikipedia: The term comes from the Latin post scriptum, an expression meaning "written after"

Thus, the explanation for why it's written P.S. is probably because it came from two separate Latin words.

Dictionary.com is wrong here. The correct Latin origin is post scriptum, literally "after the written", definitely 2 words. I've had Latin in school.

The word postscript may be based on a single latin word (the jury seems to be out on that one...), but it's not the english word postscript that you use for a postscript in a letter (or the single latin word that it might be based on), it's the latin term post scriptum, which is two words.