Yeah that kind of difference is common when you backtrack a few hundred ish years. Russian went down a track in which they added o's in kooky consonant clusters.
Compared to polish:
молоко - młoko
золото - złoto

It gets really strange when Russian starts stressing the placeholder-support vowel, like город.

o and e are generally the vowel-bolsterers for Russian. сердце from Old East Slavonic сьрдьце

https://en.wikipedia.org/wiki/Slavic..._and_pleophony
[I explained that sort of backwards actually, the correct explanation is complicated and laid out here]

Regardless, град is still a word, and has split from it's now brother word город
https://en.wiktionary.org/wiki/%D0%B...%D0%B4#Russian