Google QPX Express API isn't returning cheapest flight option

Question

While comparing the API response I got from Google's QPX Express API with the results that appear on Google's website, I noticed that more price options appear when I use the website.

For instance, my JSON response body has the flight priced at $454.26. When I look up the same flight on Google's website I am given a $455 and $193 option.

I have tried restricting the carrier, in separate requests, to Air Canada and United Airlines. In both cases I get the same, higher price.

I thought maybe the cheaper price was in American dollars but clicking through to each site shows that they are Canadian prices.

I thought maybe I was requesting too few solutions and so I put it to 500 but that still didn't return the $193 price.

I'm really not sure what I can do here. Any ideas?