The A320 FMGC will provide an optimum level based on wind, gross weight, cost index and time. If the cruise segment will last less than 15 minutes, the FMGC will recalculate an optimum level that will provide a 15 minute cruise (ie, 270 gives 10 minute cruise, FMGC will call for 205 as it will provide a 15 minute cruise).
I've not seen it written explicitly in the FCOM, but my understanding of the aerodynamics of the A320 (at least V2500 powered ones) suggests that it is more fuel efficient to climb to (for example) 270 at CLB thrust, cruise and then descend in DES mode than it is to adhere to the suggested optimum level in this situation - the argument being that the extra fuel burnt in the climb is offset by the less fuel burnt in the shorter cruise and longer descent.
On shorter sectors I tend to select the company generated flight plan level, in conjunction with my own analysis of the shear-rate and wind at different levels, up to the point that climbing too high will make my job harder. In other words, too little time in the cruise to set up the arrival!
Tricky explanations over; do whatever it takes to make your job as simple as it can be!