What's up with the grid coordinates at the Rosemont Blue Line station?

Factual Questions
1

This was discussed on chicagobus.org to no avail, and the webmaster of chicago-l doesn’t know, so I figured heck, I’ll ask here…

As many of us know, all the CTA train stops’ signs have the north/south and east/west coordinates from the Chicago street numbering system (and the Evanston numbering system on the Evanston Purple Line stops). For example, the Addsion Red Line stop’s sign tells you the coordinates are 3600N, 960W.

Now…the Rosemont Blue Line stop…when it was called “River Road,” the sign displayed the proper west coordinate – I think it was 9300 West. But when the stop was renamed “Rosemont,” the coordinate on the sign changed to 10500 West! If that coordinate were true, then that means that the station would be right at the end of an O’Hare runway!

So…why the weird coordinate number?? An e-mail to CTA got no response…

2

I don’t know but there’s a nice shot here:
http://www.panoramio.com/photo/2411489

It looks like 10500W is possible on Rosemont city streets.
Google maps link

3

Well, yeah, it is definitely possible – but at least according to Google Maps, Rosemont follows the Chicago numbering system (and in fact I believe Mannheim Road would be 10300 West)…and the system still would place the Blue Line stop at the tip of the runway…and your link does put it right where 10500 West would be…but go due south to Bryn Mawr (where the station is located) and you’ll find yourself on O’Hare turf…