Missing techs

Quick question, haven't noticed it mentioned anywhere else, but does any one know the rhyme or reason to advances not being available for selection even when you have all of the prerequisites? This can be quite annoying especially with philosophy free advance not to mention just messing up your plans for research.

Here's an explanation of the mechanics, if not a reason.

Thanks, at least I have an idea of what is going on now.

Thanks Catfish for the original reference. It is fun to read how these things were discovered in bits and pieces originally.

Irishlutheran,
The concept is called "tech hiding". You can search for it on this forum as well if you want to read more about it.

In case you have not noticed it, there are two differences between the list that is being used for tech hiding and what you get on Civilopedia. Future Tech is in the latter and not in the former. Plumbing is on the former and not in the latter.