Sativa Flowered with 14 hours light. What gives?

Hey all,
I planted outdoors this summer and was surprised to find light flowering in mid-august (according to ptaff soleil chart 13:45 hours of light at this date). This is a sativa from seed that I got in southern Africa and brought back here. I was reading Jknuc's technical essays about THC production and was surprised that this plant started flowering earlier than the 13 hour limit.

Is this normal behavior for a Sativa, or are the light charts by latitude not to be taken so literally?

http://ptaff.ca/soleil/?lang=en_CA

Yes this is normal for a sativa, they tend to be very light time sensitive. Most sativas growing close to the equator do not have much of a change from summer to winter. The plants sense the shortening days and start their flower cycle, you listen to indoor growers and they are talking sets times while outside the plants sense the light changes over the days and weeks to determine when to flower.
Enjoy your seed they are really some awesome plants down that way.

Cool, thanks for the insight.