In your case, Amir, typically the practical answer is don't worry about the 100 results per page because no one else actually searches that. I notice similar scenarios frequently, so I don't think it is unusual. When I have tried to analyze it in the past, the best explanation I can come up with is that google "nests" results differently when there is 10 per page versus 100 per page. When I'm referring to nesting listings, I'm talking about how some times there are two listings (or more) for a single domain versus just one listing. If you go through and compare the 10 result page versus 100 result page are there differences in the number of listings per domain between the two pages. I often find my page is ranked in the same location based on how many domains are above me, but the number of listings for each domain is different.
I think the algorithm is keeping changing recently from Google. I have noticed two totally different SERP for one keyword within 1 minute
I can explain that a little better, but for starters you have to understand how google works. First the algorithm is a mathematical formula. When you see different results in google, what is happening is that your query is going to different datacenters each time. Google has many datacenters in different geographic areas across the world. When I search google, I may get a result from one datacenter. When you search google, you may get a result from an entirely different datacenter. When you see results vary like you are, then likely the case is you are accessing results from two different datacenters that are not the same. When google updates their results, the first datacenter will have the results. Then those results propagate out to other datacenters until they are all the same. If all datacenters are all the same, then you will not see variances like you are describing. Then the process continues on, as google updates their algorithm or the index, etc.
you can do a quick search for google datacenters and find a variety of tools related to querying individual datacenters separately to see the results you get on each. They are referenced by IP address, and publically available.
Here is a video of Matt Cutts describing DataCenters
Here is seochat's tool for datacenters
There is a difference between Amir's scenario and AirFore1's. Amir's is due to differences in nested results. AirForce1's is due to a difference in getting different datacenters. typically, imo.