The answer is as my new boss saiz “it’s either square or it’s not”.. and I totally agree.

Out of square compounds. It’s not that hard to make things square unless you are building stuff that kinda trash to begin with. I’ve build miles and miles of cabinets “not furniture” face frame style and it’s out of square slightly like 90% of the time as there’s no real joinery. Just all screws and glue and aligning stuff the best you can by feel and with clamps..