I've added 404 error codes and no-index meta tag to pages quite some time ago. However, when I check in Google Webmaster Tools, looking at the graph of "removed" pages from their index, I see very low numbers that don't correspond with the amount of error codes and no-index tags I have on my pages. I can also reach those pages through Google Search.
I've double checked the implementation and it seems fine. Can there be another reason?
Days, Weeks and Months...
It can take Google days, weeks and even months for Google to remove pages marked noindex, robots and 404's. Generally it takes Google several crawls before Google acts on the new information of a page.
User Errors
It should be also noted that more than often users make human errors and create 404's, noindexs and so forth by error, simply removing the pages straight away would cause harm in rankings to such little mistakes, so It's my belief Google does this on purpose to ensure that these are intended changes.
Web Master Tools
Over the years many webmasters have said exactly the same as you over and over, and more times than I can count. Google made changes to Webmaster Tools several years ago allowing webmasters to quickly remove a URL by making a request. If you have a few pages that you want removed straight away then you should make a request, this is by far the quickest method and you should ensure that the pages return noindex if the page exists or a 410 gone to prevent it being re-indexed or denied removal.
410 vs 404
While a 404 is perfectly normal on a small scale it should be noted that 410 is actually preferred, this tells Google the page is GONE FOREVER! while a 404 could be used temporary, or if you have just a few pages that your not fussed about.
If you marked those pages with a noindex meta tag, there are two possible reasons I see:
The graph you are looking at in Webmaster Tools showing 'removed' I presume that is the index status graph and if so, that shows URLs removed by manual request and not through normal 404 site maintenance.
Look under Crawl > Crawl Errors and you should see recent 404 pages.
Sadly, you are taking all of the steps with the removal process. Google Webmasters also explains this issue with their YouTube video. It takes a good while for the actual removal process from all of Google and the search engine. Try to be patient now, and it will happen.
Also, you have to remember, Google is indexing, re-indexing, and deleting records from their Search Engine all of the time at a massive rate already, but there is trillions upon trillions of sites that this needs to be done to and more links for indexing.
