Why are the last 4 month names out of synch with the rest?

Question

There are twelve months, yet starting from September, the naming conventions go horribly wrong. The last 4 months use Latin numerical prefixes:

Sept = seven
Okt/Oct = eight
Nov = nine
Dec = ten

Why are these months 2 behind in their names? Did I miss the part where two additional months were added to the year?

Answer 1

Yes, originally there were only 10 months. January and February were added later. Also, July and August were originally named 5 and 6.