Same with other characters.
An obvious explanation could be magic. I mean, they can do all sorts of crazy stuff, why not some cosmetic enchantment here and there?
Not really, Varric looks more 'mature' than his DA2 version, Alistair also looks...well, 'different'. Cassandra& Leliana had changed in a right way. Morrigan looks older, but for some reasons keeps her 10-yrs old outfit. Same true for Isabela, I'm afraid. I like the face on that shot, but Pirate Queens still ignore pants and wear denim bandanas. Still, she's supposed to have 3 more or less different skins (tiers 1-4).