There are several possibilities (please note that I don't know any exact figures, so I'm just guessing here):
Performance, reliability, overall quality, cornering, speed, fuel consumption, on/offloading efficiency, capacity, width and height of vehicle (doubt there are big differences), possible customisations, servicing support (education of mechanics for example), quality/possibilities of the display on the front and the back, ...
EDIT: quite important to add: speed at which the order can be delivered. Van Hool is usually very quick I've heard.
There's many reasons why one bus would be better than another. I have no idea about the average price of a Van Hool bus, but shipping prices aren't that high any more, so they wouldn't affect it. The Dollar is quite a bit weaker than the Euro now, so that's a disadvantage (but perhaps not when they were compared/ordered/bought?).