I noticed this while reading the comics, too. I had to make the effort to stop dwelling and see past the freckles when they first appeared. Little things like that make my brain tweak. (Like the continuity lapse of Amy's gun holster in Vol. 1, at the campfire scene just before she gets bit.) In magical library somewhere, there are copies of comics in which all these accidental omissions of detail are gone.
The the explanation with which I sate myself goes something like this: Andrea and Amy are sisters (twins?). Amy was an athletic sort of girl, probably out in the sun a lot. Andrea had been an indoors, out-of-the-sun office clerk. Being in the sun gives you freckles - Amy had freckles, so most likely Andrea would also get freckles once she ditched the office. Also, a zombie-killing-vagabond lifestyle doesn't require the same personal hygiene and presentation habits as an office job. Thus, post zombie apocalypse, as Andrea becomes more active and outdoorsy, she would naturally start to model the same appearance as her late sister (freckles, less kempt hair).
So, in my mind, that's why Andrea looks like Amy did. Andrea also explains at one point how her sister was more outgoing and had more of a "life" than Andrea did, before the end of the world. Prior to apocalypse, Andrea felt boring and purposeless, however after discovering her niche, she actually has a life that she never would have found other wise (i.e. becomes more like her sister had been). And, people often begin to look like other people they admire - in this case Andrea admired some of Amy's qualities. Now that Andrea has begun living with those qualities, she may start emulating Amy, because Amy was her previous example of those qualities.
BS on my part? Yes, totally. But it works for me.
I'm pretty sure I did a double take when I saw Andrea drawn anew in Vol. 2, flipping back through Vol. 1 (TPB) to make sure I wasn't crazy or that inattentive. Maybe Charlie Adlard just has a thing for freckles on his characters in this comic. I think there's at least 2 others that have freckles. I don't think of freckles as the norm in comics, and comparatively, they seem to appear quite frequently in WD. (Of course, I might be wrong. I don't read a ton of comics. I rent these through interlibrary loan, and while up to date - besides Vol. 15 - I've only read everything once.)
Note: in both examples above "Original Amy" is Adlard's version of Amy (not Tony Moore's), which is compared to the "Original Andrea" drawn by Moore.
- That's technically not the original Amy
- Adlard was conscious of a difference between the Amy and Andrea characters, and
- Of course Adlard's versions of each sister will look similar.
While both examples still get the point across, for a better presentation, they should use Amy and Andrea as drawn by Moore in Vol. 1 (as example 2 does, showing them side by side), and then compare them to Andrea (and Amy) drawn by Adlard, and clarify who draws whom.