So you are finding some Tiff files in your Aperture library? Are your originals managed or referenced? (That is stored in the library or external to the library)
Aperture itself wouldn't be converting to Tiff but if you were using an external editor or plugin that could be returning a Tiff file to Aperture. But as you said (and as I just confirmed) if the File Type in the export preset is set to JPG then even a Tiff would be exported as JPG.
In my last post I asked if you had exported all 8 images in one batch or if you did them separately or in sub-groups, could you let me know how the export was done, it would help in figuring this out.
Are the images that were exported as Tiff also Tiff in the library? If you go back and export one of the inages that was exported as Tiff and export it again (making sure the preset File Type is really JPG) does it then export correctly?
I suppose it would be a good idea, just to eliminate anything strand in the library, for you to run a library repair. Hold the command and option (alt) keys down while starting Aperture. Select Repair library in the window that opens.