The Optimum level computed from the FMGS is computed as a function of temperature amongst others things such as CI, weight, wind etc. Any temperature deviation would affect the cruising flight level. So the differing heights of the tropopause might affect the computation.
On the A330-223 with Pratt and Whitney engines- there is is a note advising not to change the default value of 30690. As always with Airbus- no explanation why.