A commute is a two direction trip anyway and its impossible that traffic, wind direction and slope are going to be identical in both directions.

I have 12km stretch that I ocassionally do a couple times a week. Going out its always against a headwind and I`m lucky to average 12kph. On the way back, on the other hand - I can often manage 25kph.

And every day is different. Some days are so muggy you really don`t want to push that hard. So maybe keep a log and track both directions on a daily basis and chart the results. As long as the curve is slowly going up overall you`re doing good!