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

else if constexpr (IsDataTypeDecimal<RightDataType>)
                        type_res = std::make_shared<RightDataType>(right.getPrecision(), right.getScale());
                    else if constexpr (std::is_same_v<ResultDataType, DataTypeDateTime>)
                    {
                        
                        
                        
                        const TimezoneMixin * tz = nullptr;
                        if constexpr (std::is_same_v<RightDataType, DataTypeDateTime>)
                            tz = &right;
                        if constexpr (std::is_same_v<LeftDataType, DataTypeDateTime>)
                            tz = &left;
                        type_res = std::make_shared<ResultDataType>(*tz);
                    }
                    else
                        type_res = std::make_shared<ResultDataType>();
                    return true;
                }
            }
            return false;
        });
        if (valid)
            return type_res;
        throw Exception(
            "Illegal types " + arguments[0]->getName() +
            " and " + arguments[1]->getName() +
            " of arguments of function " + String(name),
            ErrorCodes::ILLEGAL_TYPE_OF_ARGUMENT);
    }
    ColumnPtr executeFixedString(const ColumnsWithTypeAndName & arguments) const