I would suspect they had the fifth A/C in maintenance and unable to operate as the back-up A/C.

The normal rotation is for four A/C to operate a 2 day cycle each with the fifth usually on stand-by/maintenance or operating ad-hoc services.

A/C 1 022 021
A/C 2 154 155 020 019
A/C 3 022 021
A/C 4 154 155 020 019
A/C 5 stand-by

The purpose of the fifth A/C on stand-by/MX was supposed to guarrantee a A345 would operate all 4 services as scheduled (which has happenned once or twice), but obviously on this occassion something else came in to play.