This is because language is contextual, and sometimes when going from Language A to Language B, some context may be lost in order to get a technically accurate translation. Then, when you use that accurate-but-context-free text that you got for Language B, and try to translate it to Language C or even back to Language A, the translation program doesn't necessarily remember or recognize the context from which it came, so you end up with a different translation.
If you like, you can post your text here and we can help out.
By the way, was my response to your other thread helpful?