I don't think it's an exception. There are two main places for the adverbial in a Swedish sentence: whatever regards the whole sentence should go after the finite verb and the subject, but other adverbials go at the end of the sentence. Like:
Jag äter inte pasta i morgon.
I will not eat pasta tomorrow.
Perfectly normal sentence.
Jag läser gärna din bok i morgon.
I'd love to read your book tomorrow
gärna is also a sentence adverbial, modifying the sentence action as a whole, so it goes in the first adverbial slot, but i morgon is just a time adverbial so it goes at the end, in the second adverbial slot.
There are some very helpful diagrams here, unfortunately in Swedish, but they show how our sentences are constructed: http://www.student.umu.se/under-studietiden/studieverkstad/skriftliga-uppgifter/skrivrad-och-sprakhjalp/ordfoljd-i-huvudsatser-och-bisatser/
When used as adverbs, they are synonyms.
But "än" can also be used in comparisons and then it cannot be replaced by "ännu": Jag är kortare än du = I am shorter than you
And "ännu" (but not "än") can also mean "even" as in "Jag är ännu kortare än du = I am even shorter than you.
This is confusing even to a Swede = Detta är förvirrande även för en svensk. (Unfortunately, "ännu" does not work here :)).