Is it normal to be annoyed by others saying "i can't complain?"

I work as a cashier in a lower-tier supermarket and part of my job is asking customers how they are doing. To which they frequently reply, "I can't complain."

I find this response to be wholly disingenuous. It not only feels like a complaint in itself , but it's also as if they're trying to take some moral high ground. As if what they were actually saying was, "although I can complain, I am choosing not to because I am blessed to even have existed."

is it normal or have I just read too far in to it?