Actually, it's not. There was a very interesting thread in this forum a couple of months ago
http://www.flyertalk.com/forum/europ...g-borders.html
where one of the participants provided a link to a Court decision
http://eur-lex.europa.eu/LexUriServ/...5CJ0241:EN:PDF
establishing that the 180-day period is fixed, not rolling.
One result of this decision is that a scenario such as the following is completely within the rules.
You enter Schengen on 1 January and leave immediately the same day.
You return to Schengen on 2 April and stay until 29 June, which will be the 90th day of presence and last of the 180-day entry period. You leave Schengen on 29 June.
You can then return on 30 June, start a new 180-day period and remain until 27 September, i.e., 90 days.
Thus you would be in Schengen for all intents and purposes continuously from 2 April until 27 September, 179 days, with only the strategic trip out on 29 June to re-set the clock.
cheers,
Henry