I guess you are talking about the Farsi words that a bilingual dictionary gives as translations of the two English words. Maybe a different bilingual dictionary would give you better definitions.
"Unspecified" means not specified, or not mentioned or not named. It doesn't necessarily mean that something is known. Your definition of "undetermined" isn't quite right, either.
not named or stated explicitly; not specified
not definitely or authoritatively decided, settled, or identified : not determined
Since both words are defined as the opposite of the same form without "un-", you can probably get a better idea of the meanings by looking for "specify" and "determine" in a few dictionaries.
Imagine a conversation to show the difference between the two words:
"I notice that you have not specified the stress level the beam can sustain." (It's unspecfied).
"That's correct, we didn't specify it because the value has not yet been determined." (It's undetermined.)