press 'Enter' and click button create different results

[sfc]

I have a question regarding the difference between clicking a button and pressing 'Enter' key. My page contains a button of name='A' within a form tag. Browser focuses on the button when the page is initially loaded. When I click the button, 'A' with it value will be put into $_REQUEST array. However, if I press 'Enter' key, which I believe it is equailvalent to clicking the button, 'A' and its value does not get into $_REQUEST.

Does anybody can give me some idea why they act differently?

Thanks for your help.