A330 Cabin altitude; Max Δp

[MD11forever]

Ok, I was thinking about this for a night and I'am stuck, maybe anybody can help on the subject.
The A330 FCOM says regarding Max Cabin Altitude:
- Max norm cabin ALT for flights shorter than 2.5 hours = 8000ft.
8000ft (752hPa/11PSI)-41000ft (179hPa/2.6PSI)=573hPa/8.4PSI

- Max norm cabin ALT for flights longer than 2.5 hours = 7350ft.
7350ft (772hPa/11.1PSI)-41000ft (179hPa/2.6PSI)=593hPa/8.5PSI

So my question is: Why is the greater differential pressure allowed on flights longer than 2.5 hours. My first thougt was as the structure gets stressed with higher Δp this should only be on short flights, but this is obviously wrong.
Any hints/clues? Thanks in advance for your help.

http://i32.tinypic.com/33mv145.png

[Meikleour]

Shorter flights therefore more Pressurisation/Depressurisation CYCLES ergo more fatigue caused!
I was on the A330 when that option became available and as far as I recall that was the rationale.

[MD11forever]

Meikleour, I see.
I was thinking on the correct topic, but not in cycles.
Shorthaul (less stress) x many
Longhaul (more stress) x less
Thank you for your answer.