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It is odd.

I could find that reason;
On ÖBB (and many European railways), some international tickets have special discounted prices like "Sparschiene" fares, kinda limited offers. So when you book, the first person might get the cheapest available fare (19.90), but there might be only one ticket left at that price.
When you add a second person, they have to get the next available fare tier (e.g. 24.90). It’s not that they charge extra for the second person on purpose — it’s just that there are only limited cheap tickets, and they sell out one by one.

Basically, the system is treating them individually, not as a group.