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

getFunctionForIntervalArithmetic(const DataTypePtr & type0, const DataTypePtr & type1, const Context & context)
    {
        bool first_is_date_or_datetime = isDateOrDateTime(type0);
        bool second_is_date_or_datetime = isDateOrDateTime(type1);
        
        if (first_is_date_or_datetime == second_is_date_or_datetime)
            return {};
        
        
        if constexpr (!is_plus && !is_minus)
            return {};
        const DataTypePtr & type_time = first_is_date_or_datetime ? type0 : type1;
        const DataTypePtr & type_interval = first_is_date_or_datetime ? type1 : type0;
        bool interval_is_number = isNumber(type_interval);
        const DataTypeInterval * interval_data_type = nullptr;
        if (!interval_is_number)
        {
            interval_data_type = checkAndGetDataType<DataTypeInterval>(type_interval.get());
            if (!interval_data_type)
                return {};
        }
        if (second_is_date_or_datetime && is_minus)
            throw Exception("Wrong order of arguments for function " + String(name) + ": argument of type Interval cannot be first.",
                ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
        std::string function_name;
        if (interval_data_type)
        {