I can see what you mean about the 5 on the left looking different, but I think some of this may be the result of over-exposure - possibly a shaft of sunlight hitting them, but more likely, I think, because they are all in lighter-coloured clothing and the photo has been exposed to bring out more detail in the darker areas.
Being hyper-critical (possibly), I wonder if this is the work of a less experienced photographer/processor. I remember watching my father printing from negatives on very primitive home-made equipment, and he would use a piece of card to obscure some areas for a time in order to prevent over-exposure. Surely a late 19th century studio would have known how to do that?
The photo and composition also seem a bit unsatisfactory to me: I think the centre of attention is meant to be the two women seated near the middle, but one of them has moved. They are placed in accordance with the rule of thirds, but slavishly following that has meant that one girl is cut off on the right - and are there others missing altogether? I think other (better?) photographers might have known how to avoid this pitfall.
Just my thoughts - and I know others are better at this than I am.