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

{
        return castTypeToEither<
            DataTypeUInt8,
            DataTypeUInt16,
            DataTypeUInt32,
            DataTypeUInt64,
            DataTypeUInt256,
            DataTypeInt8,
            DataTypeInt16,
            DataTypeInt32,
            DataTypeInt64,
            DataTypeInt128,
            DataTypeInt256,
            DataTypeFloat32,
            DataTypeFloat64,
            DataTypeDate,
            DataTypeDateTime,
            DataTypeDecimal<Decimal32>,
            DataTypeDecimal<Decimal64>,
            DataTypeDecimal<Decimal128>,
            DataTypeDecimal<Decimal256>,
            DataTypeFixedString
        >(type, std::forward<F>(f));
    }
    template <typename F>
    static bool castTypeNoFloats(const IDataType * type, F && f)