Actually, they often times have modified fuel tanks and rerated engines. The point of having an ER designation is to maintain (or increase) payload capabilities and increase range simultaneously
I was speaking of seat number/arrangement, not payload in totality.
To get my point across... show me 3 or more airlines that actually fly a 772ER with 300 seats on scheduled trips more than 5000mi... and I'll show you a surprised face!
EK is the only one I can think of.