Which Wiki page did you find that was wrong? Lots are out of date, but I found a couple that seem to say different things, and both could be correct.
The Keyboard Hot-Keys list says that "A" is "Set
Default Aspect Ratio". I can't work out when that applies, but I assume that is different to changing the Aspect ratio for just one video. It doesn't seem to change the Aspect Ration set in the MC Options/Video/Advanced.
http://wiki.jriver.com/index.php/Keyboard_Hot-keysThe Theatre View Wiki page correctly says that "a. Cycles between various aspect ratios to help stretch or zoom a picture to better fit the display."
http://wiki.jriver.com/index.php/Theater_ViewWhen you use "a" to toggle the aspect ratio, or the OSD menu via Up/Down Arrow, or right clicking on a playing video and select Window then one of the cropping or aspect ratio options, the selected Aspect Ratio information is stored in the "Playback Info" tag, so that next time you play the video it is played with the same Aspect Ratio. I don't know why that information isn't stored in the "Aspect Ratio" tag, but I assume that tag is for information only, and either comes with the media file, or is metadata collected from an external source. That is, not used for playback.
Anyway, if you remember which Wiki page you think was wrong, I could get it fixed.