The reason for the disparity is that there is a difference between getting an add-on under part 61 and part 141. Under part 141 a school may only give training for those courses for which they have received FAA approval. Add-on training is a separate course listed under Part 141, Appendix I, Additional Aircraft Category And/Or Class Rating Course. A 141 school must have FAA approval for each of these courses from the FAA. So, for example, they would need a course for commercial SE add-on, instrument add-on, etc. Many schools may not have approval for these courses, or maybe just a few of them.
If a school does not have these courses you would have to enroll in the stand alone courses which would require you to go through the entire 141 course.
Another option is to enroll in the desired course, then when you get to the point where you feel proficient to pass the check ride drop out, do a few hours with a CFI under part 61 to get the sign off, then take the check ride. In the past there was not a requirement to finish a course to get compensated by the VA. Double check on this to ensure it is still the case.
Hope this makes sense. Feel free to PM me if you have more questions.
