Oops, I see now that the dimensions are reversed: volume/length rather
than length/volume.

I have to wonder what TYPE of driving the OP did in Canada versus in
the US.  Perhaps he did lots more city driving (so gas consumption at
stops and for acceleration lowered mileage) whereas in the US the OP
did lots of highway driving or the US cities incorporated more
highways for moving around within them than the OP has in his home
area.  While it's never happened for me for more than a few minutes on
US highways because there is room to pass, I've often gotten stuck
behind a slow-moving tractor on those Candadian "highways" for a long
time so mileage suffered.

We also don't know what was the humidity in home area versus when down
in the US, or temperatures differences in the air, or even what type
of gasoline he burned at home versus in the US.  Hell, the OP might've
taken his car to the shop to have it prepped before taking a vacation,
something many vacationers do, and the timing got changed.  Maybe his
tire pressures were low and the shop upped them (or he over-inflated
them) so his tires had less rolling resistance afterward.  Tire
pressure affects mileage.  Maybe with more highway driving in the US
his tires got hotter so the pressures went up so he had less rolling
resistance.  We don't know how his driving habits changed when he
visited the US.

Another factor not often thought of (until the soaring gas prices) is
that some pumps measure inaccurately at higher temperatures.
According to federal data, Minnesota (my state) has the lowest average
temperature (at the time the fuel was dispensed).  The pumps are more
accurate at cooler temperatures.  So if the OP was fueling his car at
home when temperatures were higher then the pump's measure was more
inaccurate but in the US maybe he was filling his tank in the morning
or evening when it was cooler (than when he as refueling at home).
Gasoline expands when heated and most pumps do not adjust for
temperature.  For example, in Arizona, if you buy in 5000 gallons
increments then you end up using an adjusted pump from the
distributor, but that doesn't help the lowly motorist buying 15
gallons at a time.  I get a chuckle when I read Brendan Hawley's
comment (Canadian Petroleum Products Institute, Ottawa) who said, "it
doesn’t matter that you’re getting less volume, because you’re still
getting the same amount of energy".  Guess Hawley never took physics
to realize that with less volume (which not due to different pressure)
that there are less molecules in that reduced volume to burn up.
Motorists buy by volume, not by ergs or dynes.  While the holding tank
is underground so the temperature doesn't vary between day and night,
that presumes a full tank.  Air replaces the fuel in the tank and that
comes from above, not from some other underground cooled tank, and the
gas in the pipe and pump are obviously not underground.

While the coefficient of expansion for gasoline (0.00095 per C) is
often pooh-poohed as being insignificant, it is only when considered
alone.  Say the morning or evening were 10 C cooler than mid-day. The
volume at mid-day is 0.95% larger (so you get 0.95% less at a non-ATC
pump).  The *volume* has expanded by about 1% since the morning so you
only get 14.85 gallons which expanded into those 15 gallons that you
thought you got at noon.  At $3/gallon for that 0.15 gallons in loss
(compared to what you would've gotten at the cooler temperature), you
lost $0.45 by pumping at noon.  However, not all the gasoline you pump
into your tank will be at air temperature but then you don't know how
empty is the tank (i.e., how much atmosphere has been sucked into the
tank to warm it up versus the ground cooling it down).  Half a buck
difference per full fill might not be huge but then many folks bitch
at the McDonald's counter jockey when they're off by a nickel or even
by a penny.  A difference in dispensing temperature is just one part
of difference in mileage but not a major part.

There's always the chance that the gas stations at the OP's home are
simply ripping off their customers.  In the US, the pumps get measured
periodically.  Don't know if they check the pumps in Canada.  If
there's a chance to cheat without fear of exposure or penalty then
people will do it.