Yeah I was wondering about that myself. Thanks for explaining.

In the UK, a season is typically fewer than 10 eps, even with a half-hour show (look at the ep listings and air dates for "As Time Goes By" and "The Vicar of Dibley," both of which were popular shows but which aired, by US standards, erratically for example, sometimes more than a full calendar year passed between consecutive seasons).
My guess is that the UK implemented shorter seasons because more actors there do live theater there's much more live theater in the UK than in the US and therefore needed to commit for shorter periods of time. As US series became more and more available (popular) in the UK, demand increased for the American model (20-ish eps per 12-month period) and thus some programs began presenting multiple "seasons" per year but contractually speaking, a season is still 6-7 eps and a calendar year might feature only one season, which enables actors to blend TV, film, and stage work. (Notice that more than a year elapsed between the end of S7 and start of S8, whereas S4 thru S6 were broadcast within the same calendar year.)
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