TheMadScientist - 11:40 am on Dec 1, 2012 (gmt 0)
To expand on what he said about the 'slight difference', basically, a 404 will eventually be handled as Google handles a 410 initially.
Technically, a 410 is 'gone' and should not be re-requested by the user-agent in the future, but since webmasters change their mind and Google doesn't want to 'throw out' something good, they will re-request a 410 at a relatively infrequent rate, but the 'infrequency of re-requesting the page' begins as soon as the 410 is discovered. And, since it requires a 'deliberate action' to generate a 410, the page generating the error will be removed from the results nearly immediately.
A 404 initially is re-requested more frequently and will remain in the results for a longer period of time, because it's 'not found' and there are a number of reasons a page is 'not found' such as server error, or even some crazy 'update timing' where a site owner has decided to upload a newer version of all the pages in a directory on the server, which causes (or can cause) all the current pages in it to be deleted prior to writing the new version.
In a case such as the preceding, where there are 100 pages in the directory it can take a bit for the upload and writing to be completed, so if anyone (including Google) happens to request one (or more) of the pages after it's been deleted but before the new version is uploaded/saved, a 404 'not found' will be generated.
The page generating the error due to 'odd timing' should (correctly) be re-requested by the user-agent (including GoogleBot), and fortunately for webmasters with 'ugly upload timing', will remain in the results for a longer period than a 410 will, because a 404 Could be deliberate, but a 404 Could also be a short-term error which will be corrected, and there's no way of knowing for sure which it is from the error code, so Google 'errs on the side of caution' for 404 handling and keeps 'checking back' to see if the error is corrected at a relatively high frequency initially ... The longer a 404 goes uncorrected the less frequent the requests they make will become, and over time 404 will be re-requested at (or about) the same frequency as a 410.