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

if constexpr (result_is_decimal)
            col_res = ColVecResult::create(0, type.getScale());
        else
            col_res = ColVecResult::create(0);
        auto & vec_res = col_res->getData();
        vec_res.resize(col_left_size);
        if (col_left && col_right)
        {
            helperInvokeEither<OpCase::Vector, left_is_decimal, right_is_decimal, OpImpl, OpImplCheck>(
                col_left->getData(), col_right->getData(), vec_res, scale_a, scale_b);
        }
        else if (col_left_const && col_right)
        {
            const NativeResultType const_a = helperGetOrConvert<T0, ResultDataType>(col_left_const, left);
            helperInvokeEither<OpCase::LeftConstant, left_is_decimal, right_is_decimal, OpImpl, OpImplCheck>(
                const_a, col_right->getData(), vec_res, scale_a, scale_b);
        }
        else if (col_left && col_right_const)
        {
            const NativeResultType const_b = helperGetOrConvert<T1, ResultDataType>(col_right_const, right);
            helperInvokeEither<OpCase::RightConstant, left_is_decimal, right_is_decimal, OpImpl, OpImplCheck>(
                col_left->getData(), const_b, vec_res, scale_a, scale_b);
        }
        else
            return nullptr;
        return col_res;
    }
public:
    static constexpr auto name = Name::name;
    static FunctionPtr create(const Context & context) { return std::make_shared<FunctionBinaryArithmetic>(context); }