When selected from the Activities view, the Event Record Type is a function of the WhoId. When selected from other places it can be a function of either the WhoId, the WhatId, or a combination of the two depending on the organization and how their Record Types are set up. In this case though, Activities are all based around the account or contact (the latter has a look-up to the account), thus making the Event Record Type a function of the WhoId.
Here's some more from page 123 of the Visual Force Developer's Guide:
"Custom buttons and links are available for activities under the
individual setup links for tasks and events. However, you can override
a button that applies to both tasks and events by clicking Your Name >
Setup > Customize > Activities > Activity Buttons."
From the above, I'd conclude that its possible the org (or user) may have overridden the default properties for either the buttons or pages they're using to create tasks and/or events.
Note: There are separate buttons for Tasks and Events. Ditto for Activities. Any of the three could have custom page layouts. Events and Activities share the same "out of the box" standard page layout, which doesn't preclude an architect from creating a separate one for each of them which I've regularly seen in the orgs I've worked in.