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

ColumnPtr res;
        const bool valid = castBothTypes(left_generic, right_generic, [&](const auto & left, const auto & right)
        {
            using LeftDataType = std::decay_t<decltype(left)>;
            using RightDataType = std::decay_t<decltype(right)>;
            if constexpr (std::is_same_v<DataTypeFixedString, LeftDataType> ||
                std::is_same_v<DataTypeFixedString, RightDataType>)
            {
                if constexpr (!Op<DataTypeFixedString, DataTypeFixedString>::allow_fixed_string)
                    return false;
                else
                    return (res = executeFixedString(arguments)) != nullptr;
            }
            else
                return (res = executeNumeric(arguments, left, right)) != nullptr;
        });
        if (!valid)
        {
            
            
            throw Exception(ErrorCodes::LOGICAL_ERROR,
                "Arguments of '{}' have incorrect data types: '{}' of type '{}',"
                " '{}' of type '{}'", getName(),
                left_argument.name, left_argument.type->getName(),
                right_argument.name, right_argument.type->getName());
        }
        return res;
    }
#if USE_EMBEDDED_COMPILER
    bool isCompilableImpl(const DataTypes & arguments) const override