This is so inconsistent so you may or may not be able to reproduce it yourself, but 9 out of 10 times, when exporting a AI doc to a PSD, it will merge any layers that are overlapping each other. This results in some merged layers, while some are not merged.
I have a workaround, it work's well.
1. create a blank text layer outside of the artboard
2. duplicate this text between each layer (dupe it the layers panel by dragging and holding ALT),
3. export by "use artboard", this will trick AI into thinking the overlapping graphic layers shouldn't be merged.
This workaround is a waste of time and pretty strange that I even have to resort to a workaround to do something so simple. I was hoping that the new version would have fixed this, but it is still a problem in CS6.
Please add the ability to NOT merge layers in a future AI update. Thanks!