You'll probably get less impressions because of all the people bidding. On their side it's all about maximizing their Revenue,
So for example: lets say you were the #1 bidder the first week.
In this situation, the first impression (the first time a visitor hits the page) they'll see your ad. Depending on what your frequency cap is, your ad may show a few more times, but you'll get the first impression, so you'll get the most volume because your ad is showing first. A person, (depending on the website) may only hit the page 10-15 times, so lets say the #2 bidder gets impression #2, the #3 bidder gets impression #3 and so on.
Lets say next week, the other 100 extra people come in bidding, and they all bid higher than you.
Now you're position #101. Lets say that same visitor comes, impression #1 goes to bidder #1, etc etc, after 20 page views, the user leaves, so only 20 impressions were served to bidders #1-20, now in this case, you were in line at #101, so you won't get any impression.
Obviously there's more to take into account, like CTR, budget, etc, but in the end it's all about their eCPM. That's the simple explanation though.
