Why Cardinality not the same for rounded floats?

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I need to solve a question:

Two float values are equivalent if they return same integer with Math.round(). Why the equivalence classes arising from this equivalence relationship does not have the same cardinality?

I was thinking that cardinality is the same as you can have infinitely many numbers which would round up to the same class... For example 3.11 and 3.1111 or 3.2222 and so on would all return 3 and this could be done with any float value.