I was looking at booking:
21 Dec LHR-GLA BA1494 18:50 £45
26 Dec GLA-LCY BA8725 12:25 £52
The total when I add that up should be £97, but ba.com only wants to charge me £86.50.

Any particular reason for this abnormality or does ba.com simply not know how to add? This doesn't appear to happen with other flights on the same day.