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

explicit FunctionBinaryArithmetic(const Context & context_)
    :   context(context_),
        check_decimal_overflow(decimalCheckArithmeticOverflow(context))
    {}
    String getName() const override { return name; }
    size_t getNumberOfArguments() const override { return 2; }
    DataTypePtr getReturnTypeImpl(const DataTypes & arguments) const override
    {
        return getReturnTypeImplStatic(arguments, context);
    }
    static DataTypePtr getReturnTypeImplStatic(const DataTypes & arguments, const Context & context)
    {
        
        if (isAggregateMultiply(arguments[0], arguments[1]))
        {
            if (WhichDataType(arguments[0]).isAggregateFunction())