Ad Weights made perfect sense back at the dawn of time (2005 to 2015) before we added currency specific ads and start and end times. Now that the Ad Manager has more capabilities you need to think carefully about it and what the table shows is no longer 100% accurate. We are working towards a new UI for this but it is a challenging problem.
Weights are cumulative across all ads shown in one Hotspot.
Frequency of display is determined by the fraction of: ads weight/Hotspot's total weight. If there are 3 ads with a weight of 1, each will show 1/3 or 33% of the time.
The total weight for this Hotspot is 4 so Ad Three shows 2/4 or 50% of the time. You would see a fairly predictable distribution of ad impressions that will closely match the frequency. Ads are shown randomly so it is close but not perfect.
This is for a pretty typical dual currency USD/CAD site. Because one ad is only in USD, the CAD ad frequency is adjusted. You might also notice large discrepancies with impressions because of how your traffic is distributed between currencies.
I don't even know how to begin to discuss this. I trust you to figure it out. Call support if you have any questions.