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

}();
        const ResultType scale_a = [&]
        {
            if constexpr (IsDataTypeDecimal<RightDataType> && is_division)
                return right.getScaleMultiplier(); 
            else if constexpr (result_is_decimal)
            {
                if constexpr (is_multiply)
                    
                    
                    
                    
                    return ResultType{1};
                else
                    return type.scaleFactorFor(left, false);
            }
            else if constexpr (left_is_decimal)
            {
                if (col_left_const)
                    
                    
                    return ResultType{1};
                return 1 / DecimalUtils::convertTo<ResultType>(left.getScaleMultiplier(), 0);
            }
            else
                return 1; 
        }();
        const ResultType scale_b = [&]
        {
            if constexpr (result_is_decimal)