You are being hit by one of the great joys of the airline industry known as 'Married Segment'.
Most airlines price flights based on the origin and destination of the travel, not based on the individual flights. ie, in this case they are charging you as if you were flying from YVR to SAN, regardless of the fact you're actually flying via DFW.
The "base fare" (before taxes/etc) that you're being quoted on this flight is what's called a "through-fare" so covers both legs of the flight - YVR to DFW and DFW to SAN :
(Output from matrix.itasoftware.com) :
Fare 1: Carrier AA VVACZSN1 YVR to SAN (rules) $154.92
Passenger type ADT, one-way fare, booking code V
Covers YVR-DFW (Economy), DFW-SAN (Economy)
Note the 'booking code V' in the text above - this is what's called a fare 'bucket', and is how airlines limit the number of "cheap" seats that are available on each flight. There will be a number of seats available for sale in each fare bucket, and as those sell out prices will go up as you move to the next fare bucket. Fare buckets were historically done on the basis of an individual flight, so YVR-DFW would have it's available seats, and DFW-SAN would have it's available seats. In order to get a cheap fare all the way through, you'd have to have availability on both flights.
Now, mixing the two concepts above (origin to destination fares, and fare buckets) can cause a problem for the airline. Maybe they want to sell cheap flights from YVR-SAN (perhaps because a competitor has a sale on that route), but they don't want to offer cheap seats from YVR-DFW or DFW-SAN as there's no competition on those routes so they know they can charge a much higher price.
Thus was born the concept of "Married Segments", which is where bucket availability can be done on a pair of flights. Thus an the airline might set the number of cheap seats available on YVR-DFW and/or DFW-SAN to be zero, but at the same time have a non-zero number of cheap seats on the combined YVR-DFW-SAN flight combination.
Now with that as background, what you're actually hitting here is the opposite of what I described above! Individually, both VVR-DFW and DFW-SAN have availability in the "V" fare class for those flights. The flight booking engine is seeing this, and is giving you a price based on that availability.
However the "married segment" of YVR-DFW-SAN does NOT have availability in V class - the lowest class it has availability in is the higher "K" class. When the system gets to the point of validating the fare is valid it's realizing that there is no sufficient availability on this married segment flight, and is bumping your ticket up to a higher K fare, with a much more expensive base fare :
Fare 1: Carrier AA K0ACZNN1 YVR to SAN (rules) $312.89
Passenger type ADT, one-way fare, booking code K
Covers YVR-DFW (Economy), DFW-SAN (Economy)
Technically, this is a case of "working as expected", with the possible exception that Google Flights and the AA website should not have even offered you the cheaper price to start with for that combination of flights. You could try calling the airline and asking them to give you the lower price, but I wouldn't not be surprised if they refuse on the grounds of "no availability" of that fare on that combination of flights.
