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

{
        const IColumn & lhs_column = *arguments[0].column;
        const IColumn & rhs_column = *arguments[1].column;
        bool lhs_is_const = isColumnConst(lhs_column);
        bool rhs_is_const = isColumnConst(rhs_column);
        const ColumnAggregateFunction & lhs = typeid_cast<const ColumnAggregateFunction &>(
            lhs_is_const ? assert_cast<const ColumnConst &>(lhs_column).getDataColumn() : lhs_column);
        const ColumnAggregateFunction & rhs = typeid_cast<const ColumnAggregateFunction &>(
            rhs_is_const ? assert_cast<const ColumnConst &>(rhs_column).getDataColumn() : rhs_column);
        AggregateFunctionPtr function = lhs.getAggregateFunction();
        size_t size = (lhs_is_const && rhs_is_const) ? 1 : input_rows_count;
        auto column_to = ColumnAggregateFunction::create(function);
        column_to->reserve(size);
        for (size_t i = 0; i < size; ++i)
        {
            column_to->insertFrom(lhs.getData()[lhs_is_const ? 0 : i]);
            column_to->insertMergeFrom(rhs.getData()[rhs_is_const ? 0 : i]);
        }
        if (lhs_is_const && rhs_is_const)
            return ColumnConst::create(std::move(column_to), input_rows_count);
        else
            return column_to;
    }
    ColumnPtr executeDateTimeIntervalPlusMinus(const ColumnsWithTypeAndName & arguments, const DataTypePtr & result_type,