There could be a combination of factors like alloy type, mismatched metal types (the metal is welded with the incorrect filler) and exposure to different salts.

Quickly googling, I saw at least two dozen different grades of stainless steel that have alloy and carbon content variations. (Even aluminum comes in many different types, btw.)

For your knives, it sounds like you might be putting them in the dishwasher. At least, that's a common mistake for good cooking knives. If you are doing that, don't. The heat will gradually change the temper of the edge and make it difficult to sharpen. The metal used in those knifes may make it more prone to rust if it is not immediately dried.

If they are cheap knives, they could be using a random type of stainless that wasn't made for that purpose.

Edit: I personally don't fully buy the idea that a temper changes significantly in a dishwasher. It would take a long time to change the temper.. Like, a really long time. Metal recrystallization is a function of time and temperature'ish, but it generally needs to hit a specific temperature that dishwashers can't reach.

I'll leave the details of that to the knife makers and metallurgists. Knife makers generally say it's bad, so that is just a "safe bet".