Non-deterministic sum of floats
Actually, the link you're referring to does not say that floating point arithmetic calculations are always deterministic. In fact, in one of the answers it's mentioned that addition is not associative (meaning (a + b) + c does not necessarily equal a + (b + c)), which is also said in this answer.

If stream aggregation happens to process rows of each group in different order -- which SQL Server is usually free to do; if there is no ORDER BY in the appropriate clause, then the optimizer will choose whatever scan or seek or other query operator will be fastest, regardless of what order that performs the additions in -- then this could explain the behaviour you observe.

Addition is always deterministic: you put in the same two floats, you get the same float out.  But adding floats together in a different order can give a different result.