This is true for the obvious reason that a real computer with a big screen is just better, but it's also true for another reason: lots of vendors have been caught engaging in shady market segmentation tactics where they overcharge people on phones and/or using "apps," relative to the price offered to people on desktop browsers. It tends to be even worse for iPhone users than for Android ones, by the way.

I think the real reason a desktop (or laptop) screen is superior is really the ease with which you can open multiple browser tabs (and to a lesser extent, multiple profiles/private browsing mode) to compare prices, as well as the easy with which you can view them side-by-side instead of having to switch between different fullscreen views.