This is asymmetric routing and it is quite common.
This is because each router takes its own decision about the next-hop (i.e. the router it will send the packet to) and doesn't care (actually doesn't have any knowledge) about the path another packet in the same conversion took.
Did you ever use a GPS to go somewhere and also to do the reverse path and it brings you on a different path for the 2 travels? Or for the same path on different days it select different paths, due to accident or traffic jam?
This is the same concept: choosing the best (known) path for this particular packet right now, and it's a fundamental feature of the packet switching networks (which Internet is build on).
Note that there's an important difference between the GPS system and network routing: the GPS builds the whole path at startup, then adapts, while each router only chooses the next hop (junction) and the full path is not know in advance.