Does the standard and the strictness of the questioning at immigration vary in the different US states?
CBP is a federal agency as Ne An Derthal mentioned. As such they follow the same federally mandated guidelines at ALL postings, even the pre-clearance postings.
The only real differences are 1: the mood and mindset of the particular CBP agent you deal with, 2: the destination you are flying from.
The first is pretty hit and miss, as like ourselves CBP staff have good days and grumpy days, happy days and bored days. Just the luck of the draw.
The second reason will always come into play, as some countries are known hot beds for issues, be it ag department problems, attempts to sneak around immigration rules, etc.
Not exactly what you asked, but I think you generally get an "easier" treatment using the preclearance facilities in Ireland at Dublin or especially Shannon. I don't think you'd slip anything past them (no more than anywhere else) but the process seems smoother and friendlier. Using BA Flight 1 which stops at Shannon for preclearance is my preferred way to enter the U.S. from Europe, in part because the immigration formalities are always completed quickly and with a smile.
The questioning is no different I should say, only the presentation.
Inside the territorial US I have not really seen much difference between the different ports. Sometimes the officers are having a nice day and you can have a pleasant chat while things are processed and sometimes they are in a bad mood and it's best to keep your answers short and to the point. Same as any other job I think.