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

{
        using OpImpl = FixedStringOperationImpl<Op<UInt8, UInt8>>;
        const auto * const col_left_raw = arguments[0].column.get();
        const auto * const col_right_raw = arguments[1].column.get();
        if (const auto * col_left_const = checkAndGetColumnConst<ColumnFixedString>(col_left_raw))
        {
            if (const auto * col_right_const = checkAndGetColumnConst<ColumnFixedString>(col_right_raw))
            {
                const auto * col_left = checkAndGetColumn<ColumnFixedString>(col_left_const->getDataColumn());
                const auto * col_right = checkAndGetColumn<ColumnFixedString>(col_right_const->getDataColumn());
                if (col_left->getN() != col_right->getN())
                    return nullptr;
                auto col_res = ColumnFixedString::create(col_left->getN());
                auto & out_chars = col_res->getChars();
                out_chars.resize(col_left->getN());
                OpImpl::template process<OpCase::Vector>(
                    col_left->getChars().data(),
                    col_right->getChars().data(),
                    out_chars.data(),
                    out_chars.size(), {});
                return ColumnConst::create(std::move(col_res), col_left_raw->size());
            }
        }
        const bool is_left_column_const = checkAndGetColumnConst<ColumnFixedString>(col_left_raw) != nullptr;
        const bool is_right_column_const = checkAndGetColumnConst<ColumnFixedString>(col_right_raw) != nullptr;
        const auto * col_left = is_left_column_const
                        ? checkAndGetColumn<ColumnFixedString>(
                            checkAndGetColumnConst<ColumnFixedString>(col_left_raw)->getDataColumn())
                        : checkAndGetColumn<ColumnFixedString>(col_left_raw);
        const auto * col_right = is_right_column_const