The system does this on its own. Why, we do not know. Can we fix it? We wished!
As a general rule, we do not incorporate numbers into lessons. It might be bothersome to have to write it out in words but the sentences are often available across the czech-english/english-czech course. By requiring to spell numbers out we are teaching both languages.
For some reason this accepted the answer ""Kdo potřeboval sto dvě eura". Why? Is it ok because of 'dvě eura' being less than five and not needing genitive plural?
There are two ways of form larger numbers in Czech. One is the official translation above and the other is the one you used. For "sto dvě eura" it actually sounds fine but for other numbers like "sto dvacet jedno euro" it is less natural for me, even though perfectly allowed by the norms of the standard Czech.
I just found in a newspaper: "Vloni se u nás narodilo tisíc sto dvacet jedno dítě." I wouldn't utter this myself, I would always use "sto dvacet jedna dětí".