Well, I’ve always assumed this, but reading the paragraph really internalizes the nature of our problems (from Microsoft’s Security Blog):

Actually, Windows 95 and Windows NT 3.5 were developed almost concurrently by two separate teams.  Then, Windows NT 4.0 was developed, while a separate team developed Windows 98.  That team went on to develop Windows 98 Second Edition and Windows Me while the NT team developed Windows 2000. The teams and codebases started to really combine around the time of Windows XP’s development.  I’m leaving out some interim products like Windows 95 Service Pack 1, Windows 95 OSR2, and Windows NT 3.51, but you get the drift.