I've been studying basic usage and I found wrong minus result on DecimalXX type.

**Example Query**
`
select toDecimal64(4,4) - toDecimal32(2,2)
`

**Expected output**
2.0000

**Actual output**
3.9800

Is there anything wrong or maybe my misunderstanding of toDecimal ?

![image](https://user-images.githubusercontent.com/8475371/107513855-cdd57a00-6be3-11eb-841d-d425a198d172.png)


**How to reproduce**
1. OS: macOS Catalina 10.15.5
2. server: clickhouse-server, built from source, version 21.3.1.1
3. client:  clickhouse-client,  built from source, version 21.3.1.1

{
        if constexpr (is_division)
        {
            if constexpr (check_overflow)
            {
                bool overflow = false;
                if constexpr (!is_decimal_a)
                    overflow |= common::mulOverflow(scale, scale, scale);
                overflow |= common::mulOverflow(a, scale, a);
                if (overflow)
                    throw Exception("Decimal math overflow", ErrorCodes::DECIMAL_OVERFLOW);
            }
            else
            {
                if constexpr (!is_decimal_a)
                    scale *= scale;
                a *= scale;
            }
            return Op::template apply<NativeResultType>(a, b);
        }
    }
};
}
using namespace traits_;
using namespace impl_;
template <template <typename, typename> class Op, typename Name, bool valid_on_default_arguments = true, bool valid_on_float_arguments = true>
class FunctionBinaryArithmetic : public IFunction