That's right. The "difference" you pointed out is not a difference of the people themselves, it's a difference in classification in a completely separate system from the people.
If you do pure random assignment then having a 7 at the end of your SSN could make a difference. EX: We rolled a 10 sided die an a 7 came up. On the other hand if you want to maximizing the number of people saved other things may be considered (Age, weight, vision etc).
I am not going to say a specific system is the only correct one in that case, but rather many of the differences you suggest are meaningless could suddenly become meaningful differentiation in a moral system. Further, those differences are not necessarily inherently important outside of that specific system and situation.
Now, assigning meaning to differences is by definition part of a specific moral framework and does not generalize.
To a theoretical "hyper intelligent" 207 dimensional being we may not qualify as sentient. After-all, anything bound by something as meaningless as time is incapable of making decisions and will always respond the same to given stimuli at specific points of space-time. Such linear beings don't actually feel things they simply respond to stimuli much like plants.
AKA Same facts, different interpretation = different moral framework.
PS: Do we save women and children first, or are young warriors more important to long term survival? Same 'fact' different interpretation of that fact.