Maybe because they are using a modified form of the Hepburn romanization
As you can see here:
http://en.wikipedia.org/wiki/Hepburn_romanizationIn most cases, the long vowels are marked with an macron.
I´m not sure is this info in right, but...
Quote:
Although Hepburn is not a government standard, some government agencies mandate it. For example, the Ministry of Foreign Affairs requires the use of Hepburn on passports, and the Ministry of Land, Infrastructure and Transport requires the use of Hepburn on transport signs, including road signs and railway station signs
From the Wikipedia.
Also, you can see this versión of modified Hepburn in the signs of the railway stations in Japan
For example, in the Tokyo Station, you can see:

Please, take a look at the roman versión of the name, just below the hiragana version.
In most situation, in Japan this version of the Hepburn is used
I´m not really sure, but is my best asumption. In fact, in my experience, this system of romanization is used in all the railways signals (and i see a lot of them)