Do your pages have a date assigned by Google? Do they have a reading level assigned by Google? Let's check out WebmasterWorld!
Have you ever done a date range search and not found what you were looking for? I've chosen an unusual area in WW; "commercial exchange" for a couple examples. Try this Google search: site:www.webmasterworld.com/commercial_exchange: [google.com...] 170 or so results are shown, going to the last page of results shows actually 137 or so. Scrolling down on the first page of results shows many pages with and without dates. Now lets use Google's "search tools": Click "search tools", notice the range is "any time". Now select custom range and enter 1/1/1995 for the From date, the Too date defaults to today; [google.com...] Whoops only two results or so. Notice they do have dates! These must be the only pages that actually do have "Google" dates! Even though using other modes of search does show dates for the pages. Hmmm. I think perhaps most of these pages are Supplemental(perhaps an outdated term). I think these pages are too short!
Here's a tip: If you click on "search tools" while the toolbar is activated Google will show you the count of the results.
Note: I know there is distrust of the Google results count, but for small sites or small areas of sites, I think it works fairly well.
Now clear all the search tools and do the first search again Now select "search tools", "all results", "reading level" Notice again there are many results. Now click "search tools" again while the toolbar was still activated. About 170 results, seems reasonable. Now click the intermediate reading level. Whoops, two results! Again click "search tools" while the toolbar is activated. Hey now it shows two results or so (very convenient).
For smaller sites, say less than 500 pages you can use this trick. Do a site search, perhaps on a subdirectory if you have a large site. Again show the reading levels, you'll see percentages. Now choose a reading level: Click on "search tools" again while the toolbar is activated. The toolbar will show you the actual count for that reading level. As you've seen in this example the sum of the actual counts may come no where near the total number of results for the site search.
I've tried this on quite a few sites and many many pages do not show up in a date range search. They are probably supplemental. Likewise many pages do not have reading levels and do not show up for any reading level search. For many sites this can be 50% of the pages. I've also found when you add up the actual numbers of reading levels the total is very close too the total shown in a date range search of 1995 to today.
I did a site search on: www.webmasterworld.com/google (Google SEO) I know in the past I've had trouble finding posts when I did a date range search, posts that I knew were there! With a date range of 1/1/1995 there are only half as many results as with the no date range "any time" setting of tools.
WebmasterWorld is somewhat problematic since it is totally dynamic, but the same thing happens on static sites.