Okay, I'm pretty sure people said "2003's calendar is
more natural than 2012's", not "2003's calendar is a perfect representation of natural Ayu."
Yes, she was tan in 2003's calendar, and two of the pics are flat out horrible. But if you compare the rest side by side, the older one is obviously a lot more natural
in comparison. The makeup is more natural. The clothes are more casual. You can actually see her real hair in four of the pictures.
She just looks like such an interesting person in the 2003 calendar, even though she's tan and that year/era isn't exactly the best representation of how beautiful she is. You can tell that there's a lot of depth to her and that she doesn't have to try too hard to be what she is.
In the 2012 calendar, she is clearly trying hard to look young and interesting. The styling and makeup, which is supposed to make her look younger, is actually making her look a lot older in most of the pictures. And she is posing as hard as she possibly can which makes her look really awkward in all but one or two of the pictures.
Sorry, I just don't get why everyone automatically thinks that the word "natural" can only apply to makeup or styling or skintone. It doesn't mean that the pictures are all better or that they're all good. But they are definitely more natural, i.e. more effortless and unforced.