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

};
template <typename Op>
struct FixedStringOperationImpl
{
    template <OpCase op_case>
    static void NO_INLINE process(
        const UInt8 * __restrict a, const UInt8 * __restrict b, UInt8 * __restrict result,
        size_t size, [[maybe_unused]] size_t N)
    {
        if constexpr (op_case == OpCase::Vector)
            for (size_t i = 0; i < size; ++i)
                result[i] = Op::template apply<UInt8>(a[i], b[i]);
        else if constexpr (op_case == OpCase::LeftConstant)
            withConst<true>(b, a, result, size, N);
        else
            withConst<false>(a, b, result, size, N);
    }
private:
    template <bool inverted>
    static void NO_INLINE withConst(const UInt8 * __restrict a, const UInt8 * __restrict b, UInt8 * __restrict c, size_t size, size_t N)
    {