rerouted e -- or simply f?

There's something I don't understand about the E and F subway lines in Queens.

By way of background, for the non-Queensians, the E and F trains end (or begin, I suppose, depending on your perspective) their runs in Jamaica. After leaving Manhattan, they both follow a trunk line under Queens Blvd. Just past the Briarwood Station (which purists among us still call the Van Wyck Blvd. Station) they part course. The E goes to Jamaica Center while the F goes to 179th Street.

The E train goes closer than the F to my office in Manhattan, but the 1479 Street terminal of the F train is my home station. So on days that I work in Manhattan ("the city" in local parlance), my return commute starts with an E train but I switch to an F -- usually at Union Turnpike.

But sometimes an odd thing happens. The E train will get rerouted over the F line and go to 179th Street. When that happens, I feel like I've hit the jackpot. Instead of having to get up and wait for another train, I get to stay on the one that I'm already on. Of course, lots of other commuters are annoyed at the inconvenience for them But that's their problem.

Anyway, when the rerouting happens, we are notified by an announcement from the conductor along the lines of "This is an E train running over the F line. For Jamaica Center, please wait for the next E train." What I don't get is why they insist on calling it an "E train running over the F line" rather than an F train. Once the rerouting has occured, the train is effectively an F.

And it's not just a pedantic point I'm making. There are many immigrants in the area, some of whom aren't native English speakers. I would think that a simple announcement like "This is now an F train to 179th Street" would be easier for people to understand.

ANd don't even get me started on the occasional morning E trains that start at 179th Street...