Your clicks per day and impressions are compliment variables, but you can't have more than clicks per day than the number of your impressions. I've been experiencing constant traffic as well. Maybe it's the webmaster tool.
i'm faily sure that youre seeing this kind of data because you're probably one the first page for some big keywords but between position 5-10... you only really see the big traffic from positions 1-3 and even then you would only get a CTR of 40%
in the webmaster tools try to filter out all the other searches and focus on the data from the 'web search' to understand to full picture. and- yes...google does use some kind of limit in some situation, mainley by rank your site out of the first page.
Google Webmaster Tools isn't a tracking package, don't try and use it as one. It's a resource to check indexing and crawling issues. To track traffic you need a programme for that purpose such as Google anlaytics, Statcounter or Webtrends.
Seems you must have listings in the 8-10 spot for each impression, focus on either 1-2 place on second page or 1-3 on first page for results. I see the limiting possibility, but, found a similar issue with a few client sites. Just your positioning that is the cause.
You are correct, google decides and sends you only the predetermined number of visitors to your sites, why people get same kind of traffic everyday, why cant there be a huge fluctuation in the visits from google search engine every day, its all because google keeps a database of websites and decides who should get how much visits for the day.