When I go to do cartridge installations I always check the original geometry using a Smartractor before fitting the new cartridge. I have been horrified at how far out some were. I remember one client where a well known dealer had set his previous cartridge up using a paper card so I was told, Rega I believe. The cartridge overhang was 6mm out!
That is a point I’ve been trying to make, many setups I’ve checked have been out. With the overhang a long way out it is possible to have a setup that somehow looks close to right but in reality the setup is not great at all. Even the venerable PL-12D had an overhang aid built into it...it was imprecise but a lot better than nothing.
It's impossible to setup a cart "wrong" with a 2-null-points protractor. Not impossible, but you know what I mean - not protractor's fault.
There are many reasons why the setup would not agree with fancy protractor:
1. OK, dealer did a terrible job, possible.
2. Original dealer didn't use 2-null points protractor, but a single point protractor, but the arm distance is incorrect for some reason.
3. TT manufacturer uses different geometry than fancy protractor: e.g. Rega has its own geometry which is not exactly Baerwald nor Stevenson nor Löfgren; same goes for Roksan etc.
So cart was perhaps perfectly setup for some other geometry, doesn't mean fancy protractor is better.
4. Perhaps the dealer used cantilever position to setup the cart and you were only looking at cart body or the other way around.
5. You were both looking at cantilever, but in the meantime cantilever shifted/bent with record playback.
6. Other reasons.
Fancy protractors may be useful if you setup carts on daily or weekly basis, but are not necessary even then. IMO they don's save huge amounts of time either.
There's nothing wrong with buying fancy stuff, but fancy protractors can't help better, more precise cart installation than cheap or free 2-null-points. They can make you feel better and they do have some optical aids, that can make the job easier.