Side note. Something must have changed, because the corn is usually shoulder high by 4th of July.
In England, it’s “Shoulder High by the Late Summer Bank Holiday.”
My wife’s corn was 8’+ a month ago.
Different varieties. Most “standardized” corn varieties are shorter than a lot of the older types. That makes them less vulnerable to wind damage, and means the tractors don’t have to be 2 stories tall to get over them.
Some of the older varieties reach 15 feet easily. I’ve even seen 21’. Although there are others that are shorter. I grow 2 different types, both unruly enough to range from 8 inches tall, to 7 feet.