Originally Posted by codfishcandy

I've reached out to Sixt reservations in the past, and I wasn't always sure how well informed they were of availability in local branches. In the past at some point I was trying to convert a pay at the counter rental to a prepaid one via e-mail and I got a reply from Sixt which gave me the impression that the information that's on the site takes precedence over what they would write in an e-mail, see below.
If I were in your shoes I would either:

• Get a reservation through the site, and pre-pay it (once I'm certain it's the best rate I'm going to get) - if you walk up with a prepaid reservation which was booked through their own system they don't really have any other option than honoring it, the way I see it
• Call them (they also have a Belgian number if I recall correctly) to see what they can do, or why you can't get a reservation for the class you desire - if you reserve sufficiently in advance, and it's not high season I can't really see what the problem is (barring special circumstances, of course, such as a major conference or something)

As for why you're running into this; I presume they figure they can earn more by renting their higher end vehicles out for shorter stretches, than having someone rent it for 3 months for a neat rate. Though it would be rather disingenuous if this precludes you from renting them for that.