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

return arguments[0];
            return arguments[1];
        }
        
        if (isAggregateAddition(arguments[0], arguments[1]))
        {
            if (!arguments[0]->equals(*arguments[1]))
                throw Exception("Cannot add aggregate states of different functions: "
                    + arguments[0]->getName() + " and " + arguments[1]->getName(), ErrorCodes::CANNOT_ADD_DIFFERENT_AGGREGATE_STATES);
            return arguments[0];
        }
        
        if (auto function_builder = getFunctionForIntervalArithmetic(arguments[0], arguments[1], context))
        {
            ColumnsWithTypeAndName new_arguments(2);
            for (size_t i = 0; i < 2; ++i)
                new_arguments[i].type = arguments[i];
            
            if (WhichDataType(new_arguments[1].type).isDateOrDateTime())
                std::swap(new_arguments[0], new_arguments[1]);
            
            new_arguments[1].type = std::make_shared<DataTypeNumber<DataTypeInterval::FieldType>>();
            auto function = function_builder->build(new_arguments);
            return function->getResultType();
        }
        DataTypePtr type_res;
        const bool valid = castBothTypes(arguments[0].get(), arguments[1].get(), [&](const auto & left, const auto & right)
        {
            using LeftDataType = std::decay_t<decltype(left)>;
            using RightDataType = std::decay_t<decltype(right)>;