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

using DataTypeFromFieldType = std::conditional_t<std::is_same_v<T, NumberTraits::Error>,
    InvalidType, DataTypeNumber<T>>;
template <typename DataType> constexpr bool IsIntegral = false;
template <> inline constexpr bool IsIntegral<DataTypeUInt8> = true;
template <> inline constexpr bool IsIntegral<DataTypeUInt16> = true;
template <> inline constexpr bool IsIntegral<DataTypeUInt32> = true;
template <> inline constexpr bool IsIntegral<DataTypeUInt64> = true;
template <> inline constexpr bool IsIntegral<DataTypeInt8> = true;
template <> inline constexpr bool IsIntegral<DataTypeInt16> = true;
template <> inline constexpr bool IsIntegral<DataTypeInt32> = true;
template <> inline constexpr bool IsIntegral<DataTypeInt64> = true;
template <typename DataType> constexpr bool IsExtended = false;
template <> inline constexpr bool IsExtended<DataTypeUInt256> = true;
template <> inline constexpr bool IsExtended<DataTypeInt128> = true;
template <> inline constexpr bool IsExtended<DataTypeInt256> = true;
template <typename DataType> constexpr bool IsIntegralOrExtended = IsIntegral<DataType> || IsExtended<DataType>;
template <typename DataType> constexpr bool IsIntegralOrExtendedOrDecimal =
    IsIntegralOrExtended<DataType> ||
    IsDataTypeDecimal<DataType>;
template <typename DataType> constexpr bool IsFloatingPoint = false;
template <> inline constexpr bool IsFloatingPoint<DataTypeFloat32> = true;
template <> inline constexpr bool IsFloatingPoint<DataTypeFloat64> = true;
template <typename DataType> constexpr bool IsDateOrDateTime = false;
template <> inline constexpr bool IsDateOrDateTime<DataTypeDate> = true;
template <> inline constexpr bool IsDateOrDateTime<DataTypeDateTime> = true;
template <typename T0, typename T1> constexpr bool UseLeftDecimal = false;
template <> inline constexpr bool UseLeftDecimal<DataTypeDecimal<Decimal256>, DataTypeDecimal<Decimal128>> = true;
template <> inline constexpr bool UseLeftDecimal<DataTypeDecimal<Decimal256>, DataTypeDecimal<Decimal64>> = true;
template <> inline constexpr bool UseLeftDecimal<DataTypeDecimal<Decimal256>, DataTypeDecimal<Decimal32>> = true;
template <> inline constexpr bool UseLeftDecimal<DataTypeDecimal<Decimal128>, DataTypeDecimal<Decimal32>> = true;