We have numerous stories here in travel.se and elsewhere of people who have entered the US on vacation and were given six months validity on their B1/B2 or ESTA status.
They might have originally planned a shorter trip but seeing the extra time, they took advantage and stayed 5 months and 28 days — not overstaying but leaving just under the deadline.
But the next time they tried to come they were given a hard time or even refused another visa.
If USCIS doesn’t want you to actually stay six months, then why stamp permission for six months? Why not give those people just three months? Is it just laziness — they don’t want to adjust the date on their stamp? Or something else?