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

{
        ColumnsWithTypeAndName new_arguments = arguments;
        if (WhichDataType(new_arguments[1].type).isAggregateFunction())
            std::swap(new_arguments[0], new_arguments[1]);
        if (!isColumnConst(*new_arguments[1].column))
            throw Exception{"Illegal column " + new_arguments[1].column->getName()
                + " of argument of aggregation state multiply. Should be integer constant", ErrorCodes::ILLEGAL_COLUMN};
        const IColumn & agg_state_column = *new_arguments[0].column;
        bool agg_state_is_const = isColumnConst(agg_state_column);
        const ColumnAggregateFunction & column = typeid_cast<const ColumnAggregateFunction &>(
            agg_state_is_const ? assert_cast<const ColumnConst &>(agg_state_column).getDataColumn() : agg_state_column);
        AggregateFunctionPtr function = column.getAggregateFunction();
        size_t size = agg_state_is_const ? 1 : input_rows_count;
        auto column_to = ColumnAggregateFunction::create(function);
        column_to->reserve(size);
        auto column_from = ColumnAggregateFunction::create(function);
        column_from->reserve(size);
        for (size_t i = 0; i < size; ++i)
        {
            column_to->insertDefault();
            column_from->insertFrom(column.getData()[i]);
        }
        auto & vec_to = column_to->getData();
        auto & vec_from = column_from->getData();