Startpage and the like don’t get everything you find on Google. E.g. you can find today’s articles from even the most obscure blogs on Google. Searching for the same article won’t return that result on Startpage until at least a few weeks from now. I’m not sure if this is Google holding back on the good stuff, or if it’s Startpage not being willing to pay a higher fee.
And how many of the initial results in google will be filtered out (e.g takedown notices) when you hit the very last page? I'm doubtful about the usefulness of the results count in comparisons like this.