October seems inconsistent with September November and December Why isn't it Octember?

The last 4 months of the year were named from their Latin origins; "septem" for seven, "octo" for eight, "novem" for nine, and "decem" for ten. This was back when there were only ten months of the year instead of twelve i.e. the wintertime was mostly monthless until January and February were added to the calendar.