Each provider is going to record uniques/impressions differently. How do they define uniques and do they differ from impressions. Some parking companies provide a stat for each, an impression being ANY hit a domain receives, including bots/spiders, whereas uniques are generally used to indicate how many REAL visitors your domain receives.
It also depends what the classify as a click and again, each provider will have differing definitions of clicks. Some report EVERY click, whereas others only show unique clicks. That's not to say you aren't paid for every click, because in most cases you will be (and should be).
I have this argument all the time with people. Is it better to report every click or only uniques. I'm on the unique click fence for this reason, if you have the name, domain123.com and it receives 10 unique visitors, 9 of those users don't click a single link, but the 10th person clicks 7 links. Parking company A may report every click, giving a CTR of 70%, whereas Parking company B only reports unique clicks, thus a CTR of 10%. When analyzing your stats you'd probably think 70% CTR is good (and in most cases it would) but really the domain probably needs to be optimized better.
My guess (and that's all it is) is the high impressions is because they are counting spider/bot traffic and if they are using this stat to calculate CTR, the CTR is naturally going to be low.
So in summary, it's best to check with the parking provider how they define/calculate/record/report the different stats.
Cheers,