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

? checkAndGetColumn<ColumnFixedString>(
                            checkAndGetColumnConst<ColumnFixedString>(col_right_raw)->getDataColumn())
                        : checkAndGetColumn<ColumnFixedString>(col_right_raw);
        if (col_left && col_right)
        {
            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((is_right_column_const ? col_left->size() : col_right->size()) * col_left->getN());
            if (!is_left_column_const && !is_right_column_const)
            {
                OpImpl::template process<OpCase::Vector>(
                    col_left->getChars().data(),
                    col_right->getChars().data(),
                    out_chars.data(),
                    out_chars.size(), {});
            }
            else if (is_left_column_const)
            {
                OpImpl::template process<OpCase::LeftConstant>(
                    col_left->getChars().data(),
                    col_right->getChars().data(),
                    out_chars.data(),
                    out_chars.size(),
                    col_left->getN());
            }
            else
            {
                OpImpl::template process<OpCase::RightConstant>(