In some cases they can be different for example if you live in a foreign country and you want to change the UI language to a language that you are familiarly with, but you still want to use the date and number formats of that foreign country like:
<globalization uiCulture="es" culture="en-US" />
The other way around if you are doing business with an other country that your own, you probably want to use you own language, but change the date and number format.
And most of the time they are diffrent because you use the main culture (without the subculture) for the UI,
en instead of