Indeed my English-Portuguese reads Nevertheless - adv + conj. todavia, não obstante, contudo, apesar de.

    I think it has to do with the stronger idea conveiyed there, and obviously the words or sentences nearby.

http://www.merriam-webster.com/dictionary/nevertheless?show=0&t=1318186577
Definition of NEVERTHELESS
: in spite of that : however <her childish but nevertheless real delight — Richard Corbin>

http://www.fortunecity.com/bally/durrus/153/gramch28.html
A conjunction may be used to indicate the relationship between the ideas expressed in a clause and the ideas expressed in the rest of a sentence.
So, NEVERTHELESS in this example is a conjunction, right?
"She found him physically repugnant; nevertheless, she agreed to marry him."
   Significando aqui a despeito dela tê-lo achado repugnante, ainda que, mesmo achando....(idéia mais forte é a despeito de, contra todos as chances, não obstante - IN     SPITE OF - expressa um sentimento negativo, apesar de tudo, ela casou com ele e não comigo, ugh! )

http://www.fortunecity.com/bally/durrus/153/gramch24.html
d. Connecting adverbs
Adverbs such as however, nevertheless and therefore are often used to connect the ideas expressed by the clauses in which they occur to ideas
 expressed in previous clauses. In the following examples, the connecting adverbs are printed in bold type.

Connecting adverbs are often placed at the beginning of a clause.
e.g. I would like to go skiing. However, I have too much work to do.

Significando, contudo. (idéia mais forte é, apesar de..   isto é, uma idéia positiva, com todo esse trabalho ainda assim eu gostaria de ir! )

She was very busy; nevertheless, she found time to go swimming.
Ela estava muito ocupada, contud, achou tempo para ir nadar.
(outro idéia positiva, ainda que ocupada ela ainda achou tempo para ir nadar, que bom que ela conseguiu!.

http://www.fortunecity.com/bally/durrus/153/gramch24.html
In the first example, the adverb however, which occurs at the beginning of the clause I have too much work to do, connects the idea expressed in this clause with the idea expressed in the previous clause, I would like to go skiing. In the second example, the adverb nevertheless, which occurs at the beginning of the clause she found time to go swimming, connects the idea expressed in this clause with the idea expressed in the previous clause, she was very busy.

Many connecting adverbs may be placed in the middle position of a clause. This is often done when the clause contains no adverb of frequency.
e.g. I am, nevertheless, anxious to continue.
We thus had no difficulty finding the motel.
We have, therefore, decided to do it.

       It is a little confusing, I must admit, but I understood this way from the reading.  In fact the two concepts are very close.
       Still, as you can see below, nevertheless  can be adverb, preposition or conjuction, but this is another story... 

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Main Entry:
nevertheless  [nev-er-th
Show Spelled Part of Speech: adverb
Definition: however Synonyms: after all, although, but, even so, even though, howbeit, nonetheless, not the less, notwithstanding, regardless, still, still and all, though, withal, yet

Main Entry:
nevertheless Part of Speech: conjunction
Definition: however Synonyms: after all, besides, but, even, for all that, furthermore, howbeit, nevertheless, nonetheless, notwithstanding, still and all, though, withal, yet

Main Entry:
nevertheless  [not-with-stan-ding, -with-]
Show Spelled Part of Speech: adverb, preposition
Definition: although, however Synonyms: after all, against, at any rate, but, despite, for all that, howbeit, in any case, in any event, in spite of, nevertheless, nonetheless, on the other hand, regardless of, though, to the contrary, withal, yet