I don't know for certain but I'm hazarding a guess that the shorter ads are purely 'memory joggers'.
I'm presuming advertisers pay per minute for their products, and if a campaign has been successful there's no longer any need for the previous advert, they can now 'jog' your memory regarding that product with a cheaper shorter ad.
I could of course be talking twaddle, but it sounds good to me....lol
Yes, that's a fair logic. They probably decide at the beginning how long to run the full ad, with the shortened version to follow automatically. Cheers.
We often prepare 50, 30 and 20second versions of ads right from the start.
As mentioned above, air-time is not cheap. A campaign will often start with a lengthy ad to establish itself, with the "cut-down" ad following a little later...