A320 cruising at FL250?

[2302rs]

Hey there,

I noticed today‘s (Sept 4) OS9065 flying from Vienna (VIE) to Zakynthos (ZTH), flight time well over two hours, never climbing above FL250.
Now my question, what could be the reason for that? It was a very early morning flight, so traffic probably wasn’t a factor, and a technical problem seems unlikely as well, as the same aircraft left ZTH again an hour later or so, climbing to normal cruising levels.
Any ideas what could have caused this?

Thanks!

[AerocatS2A]

A large wind gradient with strong headwinds at high altitudes and not much wind at mid altitudes can make flying lower more efficient. Also it may have been attempt at staying out of turbulence.

[AerocatS2A]

It could still have been a technical issue. It may have been fixed during the turn around.

[Nightstop]

Could have been a FL250 operating restriction due to passenger drop down oxygen units inoperative with passengers seated in the row below said units. If no passengers seated in that row there’s no FL restriction, hence the difference outbound/inbound.

[B737900er]

I would say either technical or to avoid slots.

[2302rs]

Thank you for your input, much appreciated :)