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 ResultType = typename ResultDataType::FieldType;
        using NativeResultType = typename NativeType<ResultType>::Type;
        if constexpr (IsFloatingPoint<ResultDataType> && IsDecimalNumber<T>)
            return DecimalUtils::convertTo<NativeResultType>(col_const->template getValue<T>(), col.getScale());
        else
            return col_const->template getValue<T>();
    }
    template <OpCase op_case, bool left_decimal, bool right_decimal, class OpImpl, class OpImplCheck,
              class L, class R, class VR, class SA, class SB>
    void helperInvokeEither(const L& left, const R& right, VR& vec_res, SA scale_a, SB scale_b) const
    {
        if (check_decimal_overflow)
            OpImplCheck::template process<op_case, left_decimal, right_decimal>(left, right, vec_res, scale_a, scale_b);
        else
            OpImpl::template process<op_case, left_decimal, right_decimal>(left, right, vec_res, scale_a, scale_b);
    }
    template <class LeftDataType, class RightDataType, class ResultDataType,
              class L, class R, class CL, class CR>
    ColumnPtr executeNumericWithDecimal(