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 (left_point.isNull() || right_point.isNull())
            return {false, true, false};
        
        if (applyVisitor(FieldVisitorAccurateEquals(), left_point, right_point))
            return {true, true, false};
        const std::string_view name_view = Name::name;
        if (name_view == "minus" || name_view == "plus")
        {
            
            if (left.column && isColumnConst(*left.column))
            {
                auto transform = [&](const Field & point)
                {
                    ColumnsWithTypeAndName columns_with_constant
                        = {{left.column->cloneResized(1), left.type, left.name},
                           {right.type->createColumnConst(1, point), right.type, right.name}};
                    auto col = Base::executeImpl(columns_with_constant, return_type, 1);
                    Field point_transformed;
                    col->get(0, point_transformed);
                    return point_transformed;
                };
                transform(left_point);
                transform(right_point);
                if (name_view == "plus")
                {