Possibly to do with how the jet streams were at that time.

For longer flights, large passenger aircraft will generally aim for something like 30k feet - the thinner air means less air resistance so more fuel efficiency (the cost is the fuel required to get there, which is why I said "for longer flights").

However, at that altitude the "jet stream" can cause winds of 100+ mph. This can be helpful with your direction of travel, or it can slow you down. If the jet stream was strong and trying to blow you backwards, it may have been more efficient to travel at a lower altitude where the jet stream wasn't present.