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

NativeResultType scale_a, NativeResultType scale_b)
    {
        if constexpr (op_case == OpCase::LeftConstant) static_assert(!IsDecimalNumber<A>);
        if constexpr (op_case == OpCase::RightConstant) static_assert(!IsDecimalNumber<B>);
        size_t size;
        if constexpr (op_case == OpCase::LeftConstant)
            size = b.size();
        else
            size = a.size();
        if constexpr (is_plus_minus_compare)
        {
            if (scale_a != 1)
            {
                for (size_t i = 0; i < size; ++i)
                    c[i] = applyScaled<true>(
                        unwrap<op_case, OpCase::LeftConstant>(a, i),
                        unwrap<op_case, OpCase::RightConstant>(b, i),
                        scale_a);
                return;
            }
            else if (scale_b != 1)
            {
                for (size_t i = 0; i < size; ++i)
                    c[i] = applyScaled<false>(
                        unwrap<op_case, OpCase::LeftConstant>(a, i),
                        unwrap<op_case, OpCase::RightConstant>(b, i),
                        scale_b);
                return;
            }