You can find out more details here:
http://www.xpressbet.com/XBRewards
If you read the fine print on the "diamond" levels, it mentions that the amount wagered over the past year was used to determine your initial new XB rewards level. So even though you may have wagered more heavily the last few months, the level may be lower - if you wagered less earlier in the year.
Personally, I never really understood the old rewards program, but I knew there was some sort of threshold to get over to get any rebate at all - most months that I've wager a few hundred, I'd get nothing back - but if I had a peak and wagered a few thousand, I would get a rebate.
If you still have questions, I'd call them and ask about it. I'm not as crazy about XB's web interface, but their customer service is pretty good for the issues I've encountered.....