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 (arguments.size() == 2
            && ((arguments[0].column && isColumnConst(*arguments[0].column))
                || (arguments[1].column && isColumnConst(*arguments[1].column))))
        {
            return std::make_unique<DefaultFunction>(
                FunctionBinaryArithmeticWithConstants<Op, Name, valid_on_default_arguments, valid_on_float_arguments>::create(
                    arguments[0], arguments[1], return_type, context),
                ext::map<DataTypes>(arguments, [](const auto & elem) { return elem.type; }),
                return_type);
        }
        return std::make_unique<DefaultFunction>(
            FunctionBinaryArithmetic<Op, Name, valid_on_default_arguments, valid_on_float_arguments>::create(context),
            ext::map<DataTypes>(arguments, [](const auto & elem) { return elem.type; }),
            return_type);
    }
    DataTypePtr getReturnType(const DataTypes & arguments) const override
    {
        if (arguments.size() != 2)
            throw Exception(
                "Number of arguments for function " + getName() + " doesn't match: passed " + toString(arguments.size()) + ", should be 2",
                ErrorCodes::NUMBER_OF_ARGUMENTS_DOESNT_MATCH);
        return FunctionBinaryArithmetic<Op, Name, valid_on_default_arguments, valid_on_float_arguments>::getReturnTypeImplStatic(arguments, context);
    }
private:
    const Context & context;
};
}