Sometimes when I extend a surface, the extended areas merge with the original surface, and other times the extended areas result in different faces from the original surface. I never could figure out why extensions don't always merge with the source.
Today when I copied a moderately complex face by doing an offset of '0.00' and extended the ends of that surface, I ended up with one face. When the offset was "0.005", all of the extended areas were split into different faces from the original.
Extra faces in a model are the enemy, causing problems with many features. It's better to do the copy at '0.000', extend seamlessly, then offset that seamless face '.005' to create a seamless result than to remove the extra offset feature but be burdened with multiple faces.
Again, I don't know if this is a unique case or it applies across the board, but its something to keep an eye out for.
And here's a generally useful Tip -
Insert>Surface>Untrim can be used to extend faces too - just mess with the percentage number. Untrimmed surfaces will usually be seamless to the original, where an extension often won't be.