I came across this earlier today in the reports section and had never looked at it so I thought I'd give it a go. The results were somewhat surprising. Google defines it - "Impression share is a new AdWords metric that represents the percentage of impressions where your ads were shown out of the total available impressions in the market that you were targeting. This metric is available at the campaign and account level for search." What surprised me is that all bar one of my campaigns had a figure of well under 50%. I can account for some of this as I know that one of my campaigns has been limited by daily budget and another one is scheduled to be off on weekends. The rest however run 24/7 and never get anywhere near their daily budgets. The worst one has an impression share of 12% over the last 30 days. On the day with the highest number of clicks it used only 5% of the available daily budget. Every single keyword in the campaign is marked as great and it has been running for a few months now. So why would Google restrict the number of impressions?
I think it is the bid you have for your keywords. Sounds like more sites bid higher and get more impressions.
No I don't think so, I have bids adjusted to show all ads on page 1 pretty much across the board. With the campaign that has a 12% impression share I have 13 keywords, 6 of them have an all time average position of 1 and the lowest placed of the others is position 5.7.