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

{
    static constexpr const bool is_plus = IsOperation<Op>::plus;
    static constexpr const bool is_minus = IsOperation<Op>::minus;
    static constexpr const bool is_multiply = IsOperation<Op>::multiply;
    static constexpr const bool is_division = IsOperation<Op>::division;
    const Context & context;
    bool check_decimal_overflow = true;
    template <typename F>
    static bool castType(const IDataType * type, F && f)