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

UInt64 m = typeid_cast<const ColumnConst *>(new_arguments[1].column.get())->getValue<UInt64>();
        
        
        
        auto arena = std::make_shared<Arena>();
        column_to->addArena(arena);
        
        
        while (m)
        {
            if (m % 2)
            {
                for (size_t i = 0; i < size; ++i)
                    function->merge(vec_to[i], vec_from[i], arena.get());
                --m;
            }
            else
            {
                for (size_t i = 0; i < size; ++i)
                    function->merge(vec_from[i], vec_from[i], arena.get());
                m /= 2;
            }
        }
        if (agg_state_is_const)
            return ColumnConst::create(std::move(column_to), input_rows_count);
        else
            return column_to;
    }
    
    ColumnPtr executeAggregateAddition(const ColumnsWithTypeAndName & arguments, const DataTypePtr &, size_t input_rows_count) const