For the defense, there are three zones: the line (ends and tackles), behind the line (linebackers) and the backfield (cornerbacks and safetys.) The x-y-z defenese refers to the number of men on the line, linebackers and backfield respectively, that is 4-2-5.
The confusion arises from the fact that the "3-4" defense is really the 3-4-(4), and likewise the "4-3" is really the 4-3-(4).That is to say the the "4" is omitted in the above defensive descriptions because that is a normal number of backfielders.
The 4-2-5 is written out as such because 5 is an unusual number of backfielders. This formation is used in college football against a "passing" team, and in "pro" football in "sure" (e.g. third and long) passing situations. The defense is saying that they don't mind giving up say, a five yard run in order to stop the pass.
There were a bunch of older formations, such as the 6-2-3, or the 5-3-3, until these line-oriented defenses got beaten by passes. The only...