Could it be possible, that because there are a large number of websites advertising a particular product, they could affect how changes are made to that product?
For example, credit cards.
Some advertise 0% until May 200X…..
Some advertise 0% for 13 Months.
In order for the company advertising ‘May 200X’ to remain competitive, this needs to become ‘June 200X’ the following month.
Websites advertising the ‘0% until May 200X’ need to be updated every month in order to stay current.
So to make it easier for website owners (and, OK lets be fair, all the printed material and other advertising) to be up to date, all the time, it makes sense to advertise a fixed number of months for an introductory as opposed to a fixed offer end date.