It looks like you are comparing an Off Peak Return (
SVR) with an Off Peak
Day Return (
CDR). Of course, there
should be a CDR to Crewe, but there isn't, and as it's
not a protected fare, they don't have to have one. I guess the Birmingham to Crewe flow (including related stations) is considered a 'premium' flow as it's along a main line, and therefore they don't want people to be using CDRs.
I could give many examples of where it is cheaper to pay to go further and then to finish "short", but I won't because it would give the TOCs a list of potential anomalies to remove. Their idea of removing anomalies is generally to increase the lower fares, rather than to 'do the right thing'. In fact, I have such a list but it only gets shared with people I've met, so as to not risk getting into the TOCs hands
