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 (isAggregateMultiply(arguments[0].type, arguments[1].type))
        {
            return executeAggregateMultiply(arguments, result_type, input_rows_count);
        }
        
        if (isAggregateAddition(arguments[0].type, arguments[1].type))
        {
            return executeAggregateAddition(arguments, result_type, input_rows_count);
        }
        
        if (auto function_builder
            = getFunctionForIntervalArithmetic(arguments[0].type, arguments[1].type, context))
        {
            return executeDateTimeIntervalPlusMinus(arguments, result_type, input_rows_count, function_builder);
        }
        const auto & left_argument = arguments[0];
        const auto & right_argument = arguments[1];
        const auto * const left_generic = left_argument.type.get();
        const auto * const right_generic = right_argument.type.get();