"Achala" means "Hill".Some prefer using the "a" and "m" at the end of the word and some may not......like in English "Karn" or "Karan" becomes "Karna".
So yeah.....if one is a purist then you can use "Ratnachalam" and i have seen people using the word in it's purest form but with time the word got corrupted a bit to an extent that people refer to the word as "Ratnachal" but there are some purists who will still use the word "Ratnachalam" irrespective of the fact if it's "official" or "not".I know my answer is not perfect but broadly speaking....i guess it's just a case of "vagaries of life".