Why do I get different page view totals when selecting all browsers vs no filters?

For example, if you select all common browsers and apply the changes, you may get a total number of page hits that is less than the same query with no browsers selected. This is because you are pulling in all of those SPECIFIC common browsers and leaving out those that are not flagged as common. Some data may have come in without a "common" browser associated with it.
This scenario, along with others that are like it, are all related to how data is bucketized. This same situation can apply to Common Operating Systems, Common Devices (although unlikely), Campaign Names, Etc.
It is worth checking out the links below on why some filter options may be missing from the dropdowns. This can also lead to differences in data.

See also: My Filters are blank, but it still pulls data. How does that work?
Why don't I see my specific page name, campaign, etc in the filter drop down?