opepyxl not counting rows in spreadsheets?

[jjartus]

Prepare a spreadsheet with anything in cells A1..A5
Save as 'Myfile.xlsx?
run this code:

    import openpyxl

    wb = openpyxl.load_workbook('Myfile.xlsx')
    sheet = wb.get_sheet_by_name('My sheet')
    print(sheet.max_row)

Answer 5. OK

Now delete A5 in Myfile and save.
Run again the code.
Answer 5 (not 4). Why?