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

}
        for (size_t i = 0; i < size; ++i)
            c[i] = apply(
                unwrap<op_case, OpCase::LeftConstant>(a, i),
                unwrap<op_case, OpCase::RightConstant>(b, i));
    }
    template <bool is_decimal_a, bool is_decimal_b, class A, class B>
    static ResultType process(A a, B b, NativeResultType scale_a, NativeResultType scale_b)
    {
        static_assert(!IsDecimalNumber<A>);
        static_assert(!IsDecimalNumber<B>);
        if constexpr (is_division && is_decimal_b)
            return applyScaledDiv<is_decimal_a>(a, b, scale_a);
        else if constexpr (is_plus_minus_compare)
        {
            if (scale_a != 1)
                return applyScaled<true>(a, b, scale_a);
            if (scale_b != 1)
                return applyScaled<false>(a, b, scale_b);
        }
        return apply(a, b);
    }
private:
    static constexpr bool is_plus_minus =   IsOperation<Operation>::plus ||
                                            IsOperation<Operation>::minus;
    static constexpr bool is_multiply =     IsOperation<Operation>::multiply;
    static constexpr bool is_float_division = IsOperation<Operation>::div_floating;
    static constexpr bool is_int_division = IsOperation<Operation>::div_int ||
                                            IsOperation<Operation>::div_int_or_zero;
    static constexpr bool is_division = is_float_division || is_int_division;