Nice find! This is pricing in the GDS with a fare break in LAS. The fare basis LAS-LGW is DSZJ3NLG, with travel permitted July-Aug and Oct-May, MON/TUE/WED only
It does appear need to add an interline-ticketed segment to LAS from SFO, LAX (possibly others) on AS to get the lowest price.
I have been able to replicate on other dates. One issue is the seasonality and/or operating days of the afternoon flight from LAS-YYC to connect to the LGW flight. It does not operate every day (or at all?) for a good portion of the winter. In addition, with respect to LAX at least, there is no morning AS flight to LAS until later this year, and an overnight connection is much higher.