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 <class T>
inline constexpr const auto & undec(const T & x)
{
    if constexpr (IsDecimalNumber<T>)
        return x.value;
    else
        return x;
}
template <typename A, typename B, typename Op, typename OpResultType = typename Op::ResultType>
struct BinaryOperation
{
    using ResultType = OpResultType;
    static const constexpr bool allow_fixed_string = false;
    template <OpCase op_case>
    static void NO_INLINE process(const A * __restrict a, const B * __restrict b, ResultType * __restrict c, size_t size)
    {
        for (size_t i = 0; i < size; ++i)
            if constexpr (op_case == OpCase::Vector)
                c[i] = Op::template apply<ResultType>(a[i], b[i]);
            else if constexpr (op_case == OpCase::LeftConstant)
                c[i] = Op::template apply<ResultType>(*a, b[i]);
            else
                c[i] = Op::template apply<ResultType>(a[i], *b);
    }
    static ResultType process(A a, B b) { return Op::template apply<ResultType>(a, b); }