Investment calculator - returned number format issue

I've created a function, which returns DataFrame with 3 columns - Year, Principal amount and End balance. It works fine, however the format number of End balance is weird, but I'm not sure, what I've done wrong.

If the input is investment_calculator(15000,7,10). It returns End balance in correct format:

However if it exceeds particular amount, e.g. investment_calculator(15000,10,10). The format of End balance is different, although nothing's changed:

Function:

def investment_calculator(amount,years,interest):
    """
    Investment calculator calculates returned amount.

    Args:
        amount: monthly invested amount
        years: number of years
        interest: average yearly interest

    Returns:
        DataFrame
    """
    col_names = ['Year', 'Principal', 'End balance']
    df = pd.DataFrame(columns = col_names)

    i = 1
    while i <= years:
        invested_amount = (amount * 12) * i
        year = datetime.datetime.now().year + (i - 1)
        
        
        if i == 1:
            investment = (amount * 12) * (1 + (interest / 100))
            new_row = {'Year':year, 'Principal':invested_amount, 'End balance':investment}
            df = df.append(new_row, ignore_index=True)
        else:
            investment = (df.iloc[-1,2] + (amount * 12)) * (1 + (interest / 100))
            new_row = {'Year':year, 'Principal':invested_amount, 'End balance':investment}
            df = df.append(new_row, ignore_index=True)
            
        i += 1
        
    return df

Any ideas, what's wrong, please?